Docs: usb: update writesize, copy_from_user, usb_fill_bulk_urb, usb_submit_urb

update code examples writesize, copy_from_user, usb_fill_bulk_urb,
usb_submit_urb in skel_write() according to usb-skeleton.c

Signed-off-by: Philipp Hortmann <philipp.g.hortmann@gmail.com>
Link: https://lore.kernel.org/r/0c581a83dfc1a8c37e97dfa7279d333f367a9787.1635591623.git.philipp.g.hortmann@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>