myri10ge: fix error checking and return value in myri10ge_allocate_rings

Fix a missing error check in myri10ge_allocate_rings() and set status
to -ENOMEM before all actual allocations so that the error path returns
what it should.

Signed-off-by: Brice Goglin <brice@myri.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>