[stm32l1] Don't use an intermediate clock in setup

This switch to HSI was an attempt to run the setup code faster. However,
there's no real need for this, and it just confuses things by switching first
to one clock, and then to another.  Just keep running on the existing clock
until switching to the clock actually chosen by the user's arguments.
[stm32l1] Never write illegal values to voltage range selection

According to the reference manual, you are _forbidden_ from writing 00 to the
VOS[1:0] bits in PWR_CR.  Writing a 00 is automatically turned into range 2,
or, 10.  Attempting to then |= the bits for range 1 (01) results in the final
result of choosing voltage range 3 (11).  This has fairly catastrophic effects
if you then attempt to switch to PLL clock at 32Mhz.

Oddly, the existing code was working fine on STM32L151C6 revision W, but
failing with revision V silicon.  Regardless, the existing code was wrong and
not following the reference manual.

Further, attempting to change any power voltage range settings without the RCC
enabled will have no effect.  Arguably this should not be in the function,
but for a helper function, used elsewhere in basic sysclock bringup, I felt
that making _sure_ it was on was more useful, so that this code actually
worked as expected.