fix(st-pmic): remove deadcode from STPMIC2 driver

"regul" corresponds to a specific part of a global table that can't be
undefined. Thus, checking if it is NULL is useless.

Issue found by Coverity (CID 445089).

Signed-off-by: Maxime Méré <maxime.mere@foss.st.com>
Change-Id: Ic812bc1fde12fe8389677c7c72fb85246c50f5c9
1 file changed