pinctrl: single: Add allocation failure checking of saved_vals

Currently saved_vals is being allocated and there is no check for
failed allocation (which is more likely than normal when using
GFP_ATOMIC).  Fix this by checking for a failed allocation and
propagating this error return down the the caller chain.

Detected by CoverityScan, CID#1469841 ("Dereference null return value")
Fixes: 88a1dbde ("pinctrl: pinctrl-single: Add functions to save and restore pinctrl context")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Johan Hovold <johan@kernel.org>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>