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

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: kbuild test robot <fengguang.wu@intel.com>
Fixes: f6731485a51978ca ("tty: serial: sh-sci: Hide number of ports config question")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>