[media] em28xx: create a common function for isoc and bulk USB transfer initialization

- rename em28xx_init_isoc to em28xx_init_usb_xfer
- add parameter for isoc/bulk transfer selection which is passed to em28xx_alloc_urbs
- rename local variable isoc_buf to usb_bufs

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