staging: comedi: drivers.c: checkpatch.pl --strict cleanup

Fix the checkpatch.pl --strict issues:

CHECK: DEFINE_MUTEX definition without comment
CHECK: braces {} should be used on all arms of this statement

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>