Staging: line6: fix checkpatch errors in playback.c

2 errors left, but they are 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>