[rtl8139] Perform only 8-bit ioport access on the ChipCmd register

The ChipCmd register is only an 8-bit register.  The 16-bit access
used by iPXE was causing an issue when used with qemu emulated rtl8139
device which was improperly aligning IOs.

Signed-off-by: Julian Pidancet <julian.pidancet@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/rtl8139.c

 	}
 
 	/* Handle received packets */
-	while ( ! ( inw ( rtl->ioaddr + ChipCmd ) & RxBufEmpty ) ) {
+	while ( ! ( inb ( rtl->ioaddr + ChipCmd ) & RxBufEmpty ) ) {
 		rx_status = * ( ( uint16_t * )
 				( rtl->rx.ring + rtl->rx.offset ) );
 		rx_len = * ( ( uint16_t * )