Fix computation of timer frequency

[SnVIZQ]

Retrieving timer frequency using PwmHz::get_period() function can cause division by zero exception for certain types of frequencies which are configured by having zero in any of the PSC or ARR register - current implementation uses "clk / (psc * arr)" expression to compute the frequency.

Implementation of compute_arr_presc() sets PSC and ARR registers correctly. Performing inverse computation leads to different expression: "clk / ((psc + 1) * (arr + 1))" which properly computes timer frequency from PSC and ARR registers. This patch uses new/fixed expression for computing timer frequency.

Change log was modified.

[burrbull]

bors r+

[bors[bot]]

Build succeeded:

• ci (stm32f100, stable)
• ci (stm32f101, stable)
• ci (stm32f103, stable)
• ci (stm32f105, stable)
• ci (stm32f107, stable)
• Rustfmt