[media] hdpvr: reduce latency of i2c read/write w/recycled buffer

The current hdpvr code kmalloc's a new buffer for every i2c read and
write. Rather than do that, lets allocate a buffer in the driver's
device struct and just use that every time.

The size I've chosen for the buffer is the maximum size I could
ascertain might be used by either ir-kbd-i2c or lirc_zilog, plus a bit
of padding (lirc_zilog may use up to 100 bytes on tx, rounded that up
to 128).

Note that this might also remedy user reports of very sluggish behavior
of IR receive with hdpvr hardware.

v2: make sure (len <= (dev->i2c_buf)) [Jean Delvare]

Reported-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>