serial: sh-sci: Fix out-of-bounds access through DT alias

The sci_ports[] array is indexed using a value derived from the
"serialN" alias in DT, which may lead to an out-of-bounds access.

Fix this by adding a range check.

Note that the array size is defined by a Kconfig symbol
(CONFIG_SERIAL_SH_SCI_NR_UARTS), so this can even be triggered using a
legitimate DTB.

Fixes: 97ed9790 ("serial: sh-sci: Remove unused platform data capabilities field")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>