serial: vt8500: Fix range-checking on vt8500_uart_ports

Fix two instances where the index to vt8500_uart_ports is tested
against > VT8500_MAX_PORTS. Correct usage should be >= VT8500_MAX_PORTS.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>