staging/rdma/hfi1: fix 0-day syntax error

Setting CONFIG_HFI1_DEBUG_SDMA_ORDER causes a syntax error:
sdma.c: In function ‘complete_tx’:
sdma.c:370: error: ‘txp’ undeclared (first use in
this function)
sdma.c:370: error: (Each undeclared identifier is reported only once
sdma.c:370: error: for each function it appears in.)

Adjust code under ifdef to reference the tx properly.

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>