drivers: Remove BKL from misc_open

misc_open() is already serialized with misc_mtx. Remove the BKL
locking which got there via the BKL pushdown.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
LKML-Reference: <20091010153349.237173041@linutronix.de>