pinctrl: qcom: Handle get_function_mux failure

pinctrl: qcom: Handle get_function_mux failure

Presently, get_function_mux returns an unsigned int and cannot
differentiate between failure and correct function value. Change its
return type to int and check for failure in the caller.

Additionally, updated drivers/pinctrl/qcom/pinctrl-*.c to accommodate the
above return type change. Only compile test done.

Signed-off-by: Varadarajan Narayanan <quic_varada@quicinc.com>
Link: https://lore.kernel.org/r/20250226064505.1178054-5-quic_varada@quicinc.com
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>