mailbox: pcc: Refactor error handling in irq handler into separate function

The existing error handling logic in pcc_mbox_irq() is intermixed with the
main flow of the function. The command complete check and the complete
complete update/acknowledgment are nicely factored into separate functions.

Moves error detection and clearing logic into a separate function called:
pcc_mbox_error_check_and_clear() by extracting error-handling logic from
pcc_mbox_irq().

This ensures error checking and clearing are handled separately and it
improves maintainability by keeping the IRQ handler focused on processing
events.

Acked-by: Huisong Li <lihuisong@huawei.com>
Tested-by: Huisong Li <lihuisong@huawei.com>
Tested-by: Adam Young <admiyo@os.amperecomputing.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>