don't use BPB_TotSec16 to determine FAT type

Description for BPB_TotSec16:
> This field is the old 16-bit total count of sectors on the volume.
> This count includes the count of all sectors in all four regions of
> the volume. This field can be 0; if it is 0, then BPB_TotSec32 must
> be non-zero. For FAT32 volumes, this field must be 0. For FAT12 and
> FAT16 volumes, this field contains the sector count, and BPB_TotSec32
> is 0 if the total sector count “fits” (is less than 0x10000).
Source: https://download.microsoft.com/download/1/6/1/161ba512-40e2-4cc9-843a-923143f3456c/fatgen103.doc

TL;DR: BPB_TotSec16 can be zero even for FAT12/FAT16 volumes if the
sector count exceeds 0x10000.

Instead, let's use BPB_FATSz16. Unlike BPB_TotSec16, BPB_FATSz16 must
be non-zero for FAT12/FAT16 volumes because BPB_FATSz32 only exists in
the FAT32 BPB. According to the document mentioned above, BPB_FATSz16
must be zero for FAT32 volumes.

Author: Freax13

bios/stage-2/src/fat.rs

@@ -48,16 +48,14 @@ impl Bpb {
         let root_cluster;
         let fat_size_32;
 
-        if (total_sectors_16 == 0) && (total_sectors_32 != 0) {
+        if fat_size_16 == 0 {
             // FAT32
             fat_size_32 = u32::from_le_bytes(raw[36..40].try_into().unwrap());
             root_cluster = u32::from_le_bytes(raw[44..48].try_into().unwrap());
-        } else if (total_sectors_16 != 0) && (total_sectors_32 == 0) {
+        } else {
             // FAT12 or FAT16
             fat_size_32 = 0;
             root_cluster = 0;
-        } else {
-            panic!("ExactlyOneTotalSectorsFieldMustBeZero");
         }
 
         Self {