pinctrl: at91: fix a couple NULL vs IS_ERR() checks

The devm_kasprintf_strarray() function doesn't return NULL on error,
it returns error pointers.  Update the checks accordingly.

Fixes: f494c191 ("pinctrl: at91: use devm_kasprintf() to avoid potential leaks (part 2)")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Ryan Wanner <ryan.wanner@microchip.com>
Link: https://lore.kernel.org/r/5697980e-f687-47a7-9db8-2af34ae464bd@kili.mountain


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>