Staging: line6: fix checkpatch errors in control.c

1 error left, but it's minor.
Lots of warnings also fixed up.

Cc: Markus Grabner <grabner@icg.tugraz.at>
Cc: Mariusz Kozlowski <m.kozlowski@tuxland.pl>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>