fix(st-sdmmc2): check regulator enable/disable return

The issue was reported by Coverity [1]. The return of the functions
regulator_disable() and regulator_enable() was not checked.
If they fail, this means there is an issue either with PMIC or I2C.
The board should the stop booting with a panic().

[1] https://scan4.scan.coverity.com/reports.htm#v47771/p11439/mergedDefectId=374565



Change-Id: If5dfd5643c210e03ae4b1f4cab0168c0db89f60e
Signed-off-by: Yann Gautier <yann.gautier@st.com>