IB/qib: Fix a possible data corruption when receiving packets

Prevent a receive data corruption by ensuring that the write to update
the rcvhdrheadn register to generate an interrupt is at the very end
of the receive processing.

Signed-off-by: Ramkrishna Vepa <ram.vepa@qlogic.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@qlogic.com>
Cc: <stable@kernel.org>
Signed-off-by: Roland Dreier <roland@purestorage.com>