Kernel: Fix typo in Descriptor::set_limit()

x86 descriptor limits are 20 bytes, not 24 bytes. This was already
a 4-bit wide bitfield, so no damage done, but let's be correct.

Kernel/Arch/i386/CPU.h

@@ -81,7 +81,7 @@ union [[gnu::packed]] Descriptor
  void set_limit(u32 l)
  {
  limit_lo = (u32)l & 0xffff;
- limit_hi = ((u32)l >> 16) & 0xff;
+ limit_hi = ((u32)l >> 16) & 0xf;
  }
 };