pinctrl: zynqmp: Avoid using uninitialised variable

pinctrl: zynqmp: Avoid using uninitialised variable

In zynqmp_pinconf_set if param is PIN_CFG_IOSTANDARD or
PIN_CONFIG_POWER_SOURCE and zynqmp_pm_pinctrl_get_config returns an
error then value will not be assigned to when its value is tested to be
not equal to arg. Add code to only test value not equal to arg if ret is
false.

This issue was found by Smatch.

Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>
Link: https://lore.kernel.org/r/20250807-pinctrl_misc-v1-4-eeb564a1b032@linaro.org
Signed-off-by: Michal Simek <michal.simek@amd.com>