pinctrl: sprd: check for allocation failure

devm_pinctrl_get() could fail with ERR_PTR(-ENOMEM) so I have added a
check for that.  I also reversed the other IS_ERR() test because it was
a little confusing to test one way and then the opposite a couple lines
later.

Fixes: 41d32cfc

 ("pinctrl: sprd: Add Spreadtrum pin control driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>