mei: fix mismatch in mutex unlock-lock in mei_amthif_read()

Users of mei_amthif_read() expect it leaves dev->device_lock held,
while there is a path where mei_amthif_read() unlocks device_lock
and returns -ERESTARTSYS.

The patch move code locking device_lock back before the return.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Cc: Sedat Dilek <sedat.dilek@gmail.com>
Acked-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>