stm32f4xx: fix bug in clock config changing.
stm32f4xx: fix bug in clock config changing.

The PLL cannot be reconfigured while it's on, so if we've previously
turned it on, we must shut it off before configuration.

cbiffle/m4vgalib-demos,cbiffle/m4vgalib-demos,cbiffle/m4vgalib-demos