[media] em28xx: rename struct em28xx_usb_isoc_ctl to em28xx_usb_ctl

Also rename the corresponding field isoc_ctl in struct em28xx
to usb_ctl.
We will use this struct for USB bulk transfers, too.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>