[media] em28xx: rename isoc packet number constants and parameters

Rename EM28XX_NUM_PACKETS to EM28XX_NUM_ISOC_PACKETS and
EM28XX_DVB_MAX_PACKETS to EM28XX_DVB_NUM_ISOC_PACKETS to
clarify that these values are used only for isoc usb transfers.
Also use the term num_packets instead of max_packets, as this
is how these values are used and called in struct urb.

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