adc-stm32f0: initialize adc upon invocation of channel read
adc-stm32f0: initialize adc upon invocation of channel read

In some scenarios ADC read needs to be done before hook_init handler
take place therefore calling adc_init() directly upon invocation of
adc_read_channel, instead of waiting for the hook_init handler to
initialize adc.

BUG=b:126770302
BRANCH=None
TEST=Test together with CL:1507320.

Change-Id: I8161e8a2d0051e01804e47eb87f28b3548f1d2ac
Signed-off-by: YH Lin <a608809fda0b3a5e5ffc78f5b79106a21e96e6dd@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1515821
Reviewed-by: Daisuke Nojiri <fd5f93af191bf7e8f73ea71cc3c0f66b41b1dd49@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec