rtl8139: Fix invalid IO access alignment

This patch makes iPXE work with the rtl8139 emulation. The rtl8139
driver in iPXE issues a 16bit access on the ChipCmd register
(offset 0x37) to check the status of the rx buffer. The offset of the
ioport access was getting fixed up to 0x36 in qemu, causing the value
read in iPXE to be invalid.

This fixes an issue with iPXE reporting timeouts during TFTP transfers.

Reposting this here because it is trivial enough and the original post
on qemu-devel didn't attract much attention.

Also, the inw() which was causing the issue has been replaced with an
inb() in upstream iPXE:
https://git.ipxe.org/ipxe.git/commit/91dd64ad25baa27954a7518e73df4fca8a2d0c93

Signed-off-by: Julian Pidancet <julian.pidancet@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
