USB: g_mass_storage: code cleaned up and comments updated

Fixed most of the errors and warnings in f_mass_storage.c and
storage_common.c reported by checkpatch.pl as well as updated
comments.

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