[media] msi3101: clamp mmap buffers to reasonable level

That value is coming from the user and we need only ensure it is
reasonable. That was pointed by Hans when reviewing rtl2832_sdr driver.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Cc: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>