usb/storage: misc fixes to comments in include/linux/usb/storage.h

The fixes are:
 - fix indentation of a comment
 - fix a typo in the comment about bulk_cb_wrap.Length
 - make comment about US_BULK_CB_SIGN look like the one about
   US_BULK_CS_SIGN below
 - make the comment about bulk_cs_wrap.Signature look more like the one
   about bulk_cb_wrap.Signature

Signed-off-by: Antonio Ospite <ao2@ao2.it>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>