arch: arm: stm32f1: Fix how we determine pull-up/pull-down for gpio

When we mask out the GPIO High impedance/Pull-up/Pull-down field we
should be shifting the mask file, not shifting the field.  This is
because all the other defines already assume the shift.

Coverity-CID: 173640

Jira: ZEP-2538

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>