w1: do not unlock unheld list_mutex in __w1_remove_master_device()
w1_process_callbacks() expects to be called with dev->list_mutex held,
but it is the fact only in w1_process(). __w1_remove_master_device()
calls w1_process_callbacks() after it releases list_mutex.
The patch fixes __w1_remove_master_device() to acquire list_mutex
for w1_process_callbacks().
Found by Linux Driver Verification project (linuxtesting.org).