pinctrl: at91: Add error handling for pinctrl_utils_add_map_mux()

In atmel_pctl_dt_subnode_to_map(), the return value of
pinctrl_utils_add_map_mux() needs to be checked, for the function
will fail to associate group when the group map is full. Add error
handling for pinctrl_utils_add_map_mux() to return immediately and
propagate the error code to caller function when the function fails.

Signed-off-by: Wentao Liang <vulab@iscas.ac.cn>
Link: https://lore.kernel.org/20250318030717.781-1-vulab@iscas.ac.cn


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