bxcan: RCC enable leak

[fwolter]

In the STM32 bxcan module, the peripheral clock is enabled, but the disable was commented out in #3032 when removing the generics:

embassy/embassy-stm32/src/can/bxcan/mod.rs

Line 840 in 3e1b0e4

//rcc::disable::<T>();

@cschuhen did you leave this intentionally?

[cschuhen]

To be honest, I don't really recall. Looking at the way the code is now, I do question that whole drop method at all. Looks like for starters, from a readability POV, it's in the wrong spot of the file. I wonder if it should go into "adjust_reference_counter" instead. It should only be disabled when the last instance gets dropped.