V4L/DVB: cx23885, cx25840: Report IR max pulse width regardless of mod/demod use

Compute and report the maximum IR pulse measurment width, even
if we are set to perform carrier modulation or demodulation and
the number is fixed by the carrier freq.

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