[romprefix] Add UNDI ROMID structure bus type field

The IBM iSCSI Firmware Initiator complains that we don't have a bus
type field in the UNDI ROMID structure, so include one.

src/arch/i386/prefix/romprefix.S

 	.word	_data16_size		/* Stack segment size */
 	.word	_data16_size		/* Data segment size */
 	.word	_text16_size		/* Code segment size */
+	.ascii	"PCIR"			/* Bus type */
 	.equ undiheader_len, . - undiheader
 	.size undiheader, . - undiheader