tty: serial: sh-sci: Add default for number of ports for compile-testing

When compile-testing an allmodconfig kernel for a platform without
sh-sci serial ports, the SERIAL_SH_SCI_NR_UARTS symbol of type "int"
doesn't get assigned a numerical default value, but an empty string,
leading to a build failure:

   .config:3814:warning: symbol value '' invalid for SERIAL_SH_SCI_NR_UARTS
   ...
   make[3]: *** [silentoldconfig] Error 1

Fix this by explicitly providing a default value of 2, like before.
Reported-by: Nkbuild test robot <fengguang.wu@intel.com>
Fixes: f6731485 ("tty: serial: sh-sci: Hide number of ports config question")
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/Kconfig

@@ -766,6 +766,7 @@ config SERIAL_SH_SCI_NR_UARTS
 	default "3" if H8300
 	default "10" if SUPERH
 	default "18" if ARCH_RENESAS
+	default "2"
 
 config SERIAL_SH_SCI_CONSOLE
 	bool "Support for console on SuperH SCI(F)" if EXPERT