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

The function strict_strtoul is obsolete and should be replaced by the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

The variable midi_mask_transmit is only used as unsigned short and the
datatypes of all affected variables were adjusted accordingly.

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