Fix clock initialization to not try to use 32kHz xtal on P2_3/P2_4

None of our boards have a 32kHz xtal oscillator, instead we use those pins
(on Telemetrum) for the deployment firing circuits. The old clock
initialization code was switching from the 32kHz RC oscillator to the 32kHz
crystal and overriding our use of those pins.

Signed-off-by: Keith Packard <keithp@keithp.com>