serial: 8250: of: Correct of_platform_serial_setup() error handling

Don't dispose IRQ mapping before it has been created.

Fixes: aa959474 ("serial: 8250_of: Add IO space support")
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>