input: touchscreen: st: Fix some issues

Correct checking the proper regulator devices. Also, when
allocation of all_strbuff fails, return immediately thereby
fixing a NULL pointer access.

Change-Id: I47c980bc5491c569141c1f1b6b02c8582ddb056e
Signed-off-by: Fei Mao <feim1@codeaurora.org>