[iwm] Fix iwm_poll_bit() error value check in iwm_attach().

The iwm(4) iwm_poll_bit() function returns 1 on success, and 0 on failure,
whereas the iwl_poll_bit() in Linux iwlwifi returns < 0 on failure.

So the (ret < 0) check ended up ignoring any error returned by
iwm_poll_bit().

Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D7932