pinctrl: remove minor dead code

This removes a test whether the 'desc' variable is NULL.
This possibility has already been eliminated by the
below test earlier in the loop:

                if (desc == NULL) {
                        dev_warn(pctldev->dev,
                                 "could not get pin desc for pin %d\n",
                                 pins[i]);
                        continue;
                }

Found with Coverity: CID #1090078

Signed-off-by: Michael Opdenacker <michael.opdenacker@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>