pmic: stpmic1: set stpmic1_is_regulator_enabled() as boolean

Improve use and readability.

Change-Id: Ia99fc38287f36c9dd12bfe51352afa5da68c0e47
Signed-off-by: Nicolas Le Bayon <nicolas.le.bayon@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/142979


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: CIBUILD <smet-aci-builds@lists.codex.cro.st.com>
Reviewed-by: Yann GAUTIER <yann.gautier@st.com>