if_bnxt: Fix the issue when kernel invokes _init() again without _stop()

Fixed the issue when kernel invokes _init() when it is already in
_init() state by invoking _stop() in such case.

Reviewed by: imp
Differential Revsion: https://reviews.freebsd.org/D36445

(cherry picked from commit baef1301a2bc60be3908f44dff1f83dbbd8f812a)