tty: serial: samsung_tty: support 18 uart ports

ExynosAutov920 SoC supports 18 UART ports, update
the value of UART_NR to accommodate the same.

Signed-off-by: Faraz Ata <faraz.ata@samsung.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250429102941.4138463-1-faraz.ata@samsung.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>