* iwm_poll_bit() returns 1 on success and 0 on failure, whereas
iwl_poll_bit() in Linux's iwlwifi returns >= 0 on success and < 0 on
failure.
* Because of the wrong iwm_poll_bit return code check, no warning was
printed if tx DMA stopping failed.
Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D7371