[iwm] Fix iwm_poll_bit() usage in iwm_stop_device(), fixup r303418.

• 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