drivers/infiniband/hw/mthca/mthca_qp: kill uninit'd var warning

drivers/infiniband/hw/mthca/mthca_qp.c: In function
  ‘mthca_tavor_post_send’:
drivers/infiniband/hw/mthca/mthca_qp.c:1594: warning: ‘f0’ may be used
  uninitialized in this function
drivers/infiniband/hw/mthca/mthca_qp.c: In function
  ‘mthca_arbel_post_send’:
drivers/infiniband/hw/mthca/mthca_qp.c:1949: warning: ‘f0’ may be used
  uninitialized in this function

Initializing 'f0' is not strictly necessary in either case, AFAICS.

I was considering use of uninitialized_var(), but looking at the
complex flow of control in each function, I feel it is wiser and
safer to simply zero the var and be certain of ourselves.

Signed-off-by: Jeff Garzik <jeff@garzik.org>