media: lirc: no need to recalculate duration

This is code existed for when drivers would send less than the whole
buffer; no driver does this any more, so this is redundant. Drivers
should return -EINVAL if they cannot send the entire buffer.

Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>