Add bounds check on PLL R/Q dividers

[richardeoin]

Register fields are 7-bit.

Closes #406

[richardeoin]

bors r+

[bors[bot]]

Build succeeded:

• ci (1.61.0, stm32h735)
• ci (1.61.0, stm32h743)
• ci (1.61.0, stm32h743v)
• ci (1.61.0, stm32h747cm7)
• ci (1.61.0, stm32h753)
• ci (1.61.0, stm32h753v)
• ci (1.61.0, stm32h7b0)
• ci (1.61.0, stm32h7b3)
• ci (stable, stm32h735)
• ci (stable, stm32h743)
• ci (stable, stm32h743v)
• ci (stable, stm32h747cm7)
• ci (stable, stm32h753)
• ci (stable, stm32h753v)
• ci (stable, stm32h7b0)
• ci (stable, stm32h7b3)
• Rustfmt