Reconfiguring Clocks results in endless loop

[jhbruhn]

When initializing the clocks again, the device gets stuck in an endless loop during clocks freeze():

stm32l4xx-hal/src/rcc.rs

Line 738 in e3a1333

while rcc.cr.read().pllrdy().bit_is_set() {}

This scenario is relevant in bootloader + application scenarios for example, where the application initializes the clocks independently from the bootloader.