IB/mthca: No userspace SRQs if HCA doesn't have SRQ support

Leave all SRQ methods out of the device's uverbs_cmd_mask if the
device doesn't have SRQ support (because of ancient firmware) so that
we don't allow userspace to call the driver's create_srq method.  This
fixes a userspace-triggerable oops caused by ib_uverbs_create_srq()
following the device's ->create_srq function pointer, which will be
NULL if the device doesn't support SRQs.

Signed-off-by: Roland Dreier <rolandd@cisco.com>