staging: sep: call to sep_ioctl() may leave driver in unusable state

If sep_ioctl() is called from a process that does not own
current transaction, it unlocks unheld sep->ioctl_mutex and
returns -EACCES leaving sep->sep_mutex acquired.

The patch fixes the mutex lock-unlock mismatch.

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

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>