Update drv_pwm.c

Detection & fix for the "8 channels in 18ms Frsky problem" see http://diydrones.com/profiles/blogs/why-frsky-cppm-signal-is-so-disappointing . Note: failsafeCheck not altered!
I thought BF wants it that way because here: https://github.com/multiwii/baseflight/blob/master/src/mw.c#L108