NULL is used to designate a dummy regulator, so it it should be safe
to use against regulator_enable(). Any value that would retrun true
for IS_ERR(), OTOH, is not. Such value would also pass "if (r)" check
without any problems.

Fix the code to use !IS_ERR() instead.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>