serial: ar933x_uart: Fix off-by-one for checking valid alias id

Current code uses the alias id as array subscript of ar933x_console_ports.
So the valid id is 0 ... CONFIG_SERIAL_AR933X_NR_UARTS - 1.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>