reorder ops in sk_start to avoid possibly rolling back ifq_deq.

reorder ops in sk_start to avoid possibly rolling back ifq_deq.

instead of checking for space after beggining to deq a packet, check
for space before committing to handling a packet. this means we
can use ifq_dequeue instead of ifq_deq_begin/commit/rollback.

this is a port of src/sys/dev/pci/if_sk.c r1.187, but tweaked to
account for msk using twice the tx ring descriptors thanks to its
annoying support for 64bit addresses.

tested on an od1000
ok jmatthew@ sthen@