add a `.set_count(count_value)` method to `Qei`

[eZioPan]

There will be 2 benifit of it.

1. When users call .set_count() on Qei setup, underlying TIM CNT can have value like 0x8_000(for 16bit TIM) or 8_000_000 (for 32bit TIM), thus greatly avoid TIM underflow. Currently CNT of TIM set to 0 at begining, if user try decrease count, then CNT will read as (2^16) - x or (2^32) - x.
2. Some Rotary Encoders use its shaft as a push button. When it pushed, we can use .set_count() clear/set CNT value. (eg. a volume thumbwheel to set volume to default).

I do a #677 , but there seems some weird problems. I guess the unsafe {} may related to PAC crate?
Don't know if it's a good idea to make a PR with this code quality.
Anyway, I make a PR.