FreeBIOS: Ensure upper 16 bits are cleared in the initial crc16 value.

Fixes Castlevania: Dawn of Sorrow's checksumming which uses crc16 swi and has garbage in the upper 16 bits of r0.
The official BIOS would seem to implicitly clear these upper 16 bits.

freebios/bios_common.S

@@ -517,6 +517,9 @@ swi_get_crc16:
   mov const_0x1E, #0x1E
   adr crc_table_ptr, crc_table
 
+  bic crc_value, crc_value, #0xFF000000
+  bic crc_value, crc_value, #0x00FF0000
+
   movs crc_length, crc_length, lsr #1
   beq 1f