media: mtk-cir: fix calculation of chk period

Since commit 528222d8 ("media: rc: harmonize infrared durations to
microseconds"), the calculation of the chk period is wrong. As a result,
all reported IR will have incorrect timings.

Now that the calculations are done in microseconds rather than nanoseconds,
we can fold the calculations in a simpler form with less rounding error.

Tested-by: Frank Wunderlich <frank-w@public-files.de>
Fixes: 528222d8 ("media: rc: harmonize infrared durations to microseconds")
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>