[media] gspca - main: rename build_ep_tb to build_isoc_ep_tb

After the "gspca: Fix bulk mode cameras no longer working (regression fix)"
patch build_ep_tb is no longer being called for bulk endpoints.

This patch renames build_ep_tb to build_isoc_ep_tb to make clear it should
only be called for isoc mode cameras.

This patch also:
- drops the no longer needed xfer parameter
- removes a check for bulk mode from the build_isoc_ep_tb code

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jean-François Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>