cam: don't lock while handling an AC_UNIT_ATTENTION

Don't take the device_mtx lock in daasync() when handling an
AC_UNIT_ATTENTION. Instead, assert the lock is held before modifying the
periph's softc flags.

The device_mtx lock is taken in xptdevicetraverse() before daasync()
is eventually called in xpt_async_bcast().

PR: 240917, 226510, 226578
Reviewed by: imp
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D27735