USB: gadget: storage_common: comments updated

Updated comment to describe why printing macros are needed even
thought they are copied form the composite.h.  Also, made multiline
comments follow the coding standard.

Signed-off-by: Michal Nazarewicz <m.nazarewicz@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>