RDMa/mthca: Work around -Wenum-conversion warning

RDMa/mthca: Work around -Wenum-conversion warning

gcc points out a suspicious mixing of enum types in a function that
converts from MTHCA_OPCODE_* values to IB_WC_* values:

drivers/infiniband/hw/mthca/mthca_cq.c: In function 'mthca_poll_one':
drivers/infiniband/hw/mthca/mthca_cq.c:607:21: warning: implicit conversion from 'enum <anonymous>' to 'enum ib_wc_opcode' [-Wenum-conversion]
  607 |    entry->opcode    = MTHCA_OPCODE_INVALID;

Nothing seems to ever check for MTHCA_OPCODE_INVALID again, no idea if
this is meaningful, but it seems harmless as it deals with an invalid
input.

Remove MTHCA_OPCODE_INVALID and set the ib_wc_opcode to 0xFF, which is
still bogus, but at least doesn't make compiler warnings.

Fixes: 2a4443a69934 ("[PATCH] IB/mthca: fill in opcode field for send completions")
Link: https://lore.kernel.org/r/20201026211311.3887003-1-arnd@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>