staging/fwserial: (coding style) remove not needed return statements

Style-only modifications to make checkpatch.pl -f a bit happier.
Fixes warning: "void function return statements are not generally useful"

Signed-off-by: Radek Dostal <rd@radekdostal.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>