RDMA/uverbs: Convert command mask validity check function to be bool

The function validate_command_mask() returns only two results: success
or failure, so convert it to return bool instead of 0 and -1.

Reported-by: Noa Osherovich <noaos@mellanox.com>
Reviewed-by: Matan Barak <matanb@mellanox.com>
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>