[undi] Scan for UNDI ROMs on 512-byte boundaries rather than 2kB boundaries

Apparently some BIOSes will place option ROMs on 512-byte boundaries.
While this is against specification, it doesn't actually hurt
anything, so we may as well increase our scan granularity to 512
bytes.

Contributed by Luca <lucarx76@gmail.com>

src/arch/i386/drivers/net/undirom.c

 
 	DBG ( "Scanning for PXE expansion ROMs\n" );
 
-	/* Scan through expansion ROM region at 2kB intervals */
+	/* Scan through expansion ROM region at 512 byte intervals */
 	for ( rom_segment = 0xc000 ; rom_segment < 0x10000 ;
-	      rom_segment += 0x80 ) {
+	      rom_segment += 0x20 ) {
 		undirom_probe ( rom_segment );
 	}