firmware: psci: Fix return value from psci_system_suspend()

Currently we return the value from invoke_psci_fn() directly as return
value from psci_system_suspend(). It is wrong to send the PSCI interface
return value directly. psci_to_linux_errno() provide the mapping from
PSCI return value to the one that can be returned to the callers within
the kernel.

Use psci_to_linux_errno() to convert and return the correct value from
psci_system_suspend().

Fixes: faf7ec4a

 ("drivers: firmware: psci: add system suspend support")
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Link: https://lore.kernel.org/r/20240515095528.1949992-1-sudeep.holla@arm.com
Signed-off-by: Arnd Bergmann <arnd@arndb.de>