rtl8139: fix regression in TxStatus/TxAddr read

rtl8139: fix regression in TxStatus/TxAddr read

Commit afe0a595356192 added byte reads for TxStatus/TxAddr, but
broke 32-bit reads; the mask generation

   (1 << (8 * size)) - 1

is unspecified in C for size >= sizeof(int), and in fact returns 0
on x86.

Fix by using a larger type.

Fixes (at least) Fedora 9 i386 with -machine kernel_irqchip=on.  I
didn't see it with the qemu APIC implementation; may be due to timing
or (more likely) a tester error.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>