[media] m88ds3103: calculate DiSEqC message sending time

DiSEqC message sending takes 13.5 ms per byte, which is 54 ms total
when typical 4 byte message is sent. Don't hard-code time limit to
54 ms, but calculate it. Time limit is only used to determine when to
start poll "DiSEqC Tx ready" status from the chip.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>