serial: 8250_hp300: trivial: fix symbol name in #warning message

The symbol is defined in drivers/tty/serial/8250/Kconfig as
"SERIAL_8250", not just "8250".

Signed-off-by: Daniele Forsi <dforsi@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>