drivers: firmware: Handle error case in the zynqmp_pm_feature

drivers: firmware: Handle error case in the zynqmp_pm_feature

Unhandled error coming from xilinx_pm_request() but return
value is not read back that's why getting sparse warning
as below:
warning: variable 'ret' set but not used [-Wunused-but-set-variable].
In case of error return the "ret" value.

Signed-off-by: Venkatesh Yadav Abbarapu <venkatesh.abbarapu@amd.com>
Link: https://lore.kernel.org/r/20231011025647.17200-1-venkatesh.abbarapu@amd.com
Signed-off-by: Michal Simek <michal.simek@amd.com>