V4L/DVB (13119): gspca - pac7311: remove magic value for USB_BUF_SZ

The length check in reg_w_var() function is because the buffer contents
is copied to gspca_dev->usb_buf which has the size of USB_BUF_SZ bytes as
defined in drivers/media/video/gspca/gspca.h. Replace the number with
symbol for better readability and maintainability.

Signed-off-by: Marton Nemeth <nm127@freemail.hu>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>