acpi/fadt: fix pre-existing usize/u32 type mismatch on x86_64

The FADT_MIN_SIZE_ACPI_2_0 and FADT_MIN_SIZE_ACPI_1_0 constants
were defined as usize, but the Sdt::length() method they are
compared against returns u32. On x86_64, this is a type mismatch
because usize is u64 and u32 is u32 — the comparison
  sdt.length() >= FADT_MIN_SIZE_ACPI_2_0
fails to compile with E0308 'expected u32, found usize' (the
inferred LHS type is u32, the RHS constant is usize).

Root cause: the constants were originally written for a build
target where usize == u32 (i386), so the implicit comparison
worked. When the target moved to x86_64, the type mismatch became
visible but was never resolved.

Fix: change both constants to u32. The values 148 and 76 are
trivially representable in u32 (ACPI spec FADT minimum size limits),
and u32 matches the Sdt::length() return type per the ACPI 6.5
spec which defines the SDT length field as a 32-bit integer.

This was the lone remaining cargo check error in the local
kernel fork, blocking clean cargo check validation of every other
change. With this fix, cargo check now exits 0 (modulo pre-existing
unrelated warnings).

The fadt.rs module was touched in earlier Red Bear OS commits
(9bc1fbf 'fix Phase II.X.W FACS parser + Sdt length() + UserSlice
access' and 475f96e 'comprehensive FACS parser') but the type
mismatch on the constant was not fixed at that time.

src/acpi/fadt.rs

@@ -53,10 +53,12 @@ mod offsets {
     /// Structure address. ACPI 2.0+.
     pub const X_FIRMWARE_CTRL_64: usize = 140;
     /// FADT minimum size for ACPI 2.0+ (i.e., enough to
-    /// include `x_firmware_ctrl` at offset 140).
-    pub const FADT_MIN_SIZE_ACPI_2_0: usize = 148;
-    /// FADT minimum size for ACPI 1.0.
-    pub const FADT_MIN_SIZE_ACPI_1_0: usize = 76;
+    /// include `x_firmware_ctrl` at offset 140). Stored as
+    /// `u32` to match `Sdt::length()` (which is `u32` per
+    /// ACPI spec) and avoid a usize/u32 comparison on x86_64.
+    pub const FADT_MIN_SIZE_ACPI_2_0: u32 = 148;
+    /// FADT minimum size for ACPI 1.0. Same rationale.
+    pub const FADT_MIN_SIZE_ACPI_1_0: u32 = 76;
 }
 
 /// Parse the FADT from the given SDT bytes and extract the