Don't unlock mutex in interrupt thread (except for wpi_rx_done() - for now) -
fixes various races between wpi_notif_intr() and wpi_stop_locked().

(attachment 154381)

Committers note: yes, unlock/if_input/lock has to go away, but that'll
have to be done later.

PR: kern/197143
Submitted by: Andriy Voskoboinyk <s3erios@gmail.com>