V4L/DVB: ivtv: Fix ivtv_api_get_data() to avoid unneeded IO during IRQ handling

ivtv_api_get_data() was performing more PCI MMIO than needed, resulting
often in it accounting for more than half the total time spent in
ivtv_irq_handler().  Now it only reads at most 7 of the 16 mailbox data words
over the PCI bus, and in some instances only 2 or 3 data words as needed.

Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>