Staging: line6/midi.c: Fixed call of obsolete function strict_strtoul

The obsolete function strict_strtoul should be replaced by the kstrto*
functions.

In this context kstrtou16 should be use, as midi_mask_receive is only
used as unsigned short. All corresponding datatypes were adapted
accordingly.

Signed-off-by: Andor Daam <andor.daam@googlemail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>