[MTD] Always initialise mutex in new mtd_blktrans_dev.

We were only initialising the mutex in the case where the new device was
automatically allocated the highest minor number. If the caller
specified a minor number, or if it filled in a free slot which was made
by a previous device deregistering, the mutex wouldn't get initialised
when we jumped out of the loop.

Reported by Monte Copeland <catboat@texas.net>

Signed-off-by: David Woodhouse <dwmw2@infradead.org>