[pxe] Update UNDI transmit count before transmitting packet
It is possible that the UNDI ISR may be triggered before netdev_tx()
returns control to pxenv_undi_transmit(). This means that
pxenv_undi_isr() may see a zero undi_tx_count, and so not check for TX
completions. This is not a significant problem, since it will check
for TX completions on the next call to pxenv_undi_isr() anyway; it
just means that the NBP will see a spurious IRQ that was apparently
caused by nothing.
Fix by updating the undi_tx_count before calling netdev_tx(), so that
pxenv_undi_isr() can decrement it and report the TX completion.