In case of an optional regulator missing, regulator core will return
ERR_PTR(-ENOENT) and not NULL, so the check for missing regulator is
incorrect. Also, the regulator is not optional, it may simply be missing
from platform description, so let's use devm_regulator_get() and rely on
regulator core to give us dummy supply when real one is not available.

Fixes: d257f298 ("Input: tsc2005 - convert to gpiod")
Acked-By: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>