[PATCH] smc91x: fix one source of spurious interrupts

Not only SMC_ACK_INT(IM_TX_EMPTY_INT) in in smc_hardware_send_pkt)
appears to be unnecessary (tested with an SMC91C94 and SMC91C111), but
it seems to trigger spurious interrupts on some machines as well.
Removed.

While at it, let's log any remaining spurious interrupts if any (and
clean usage of the max IRQ loop count value).

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>