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>

drivers/tty/serial/8250/8250_hp300.c

@@ -21,7 +21,7 @@
 #include "8250.h"
 
 #if !defined(CONFIG_HPDCA) && !defined(CONFIG_HPAPCI)
-#warning CONFIG_8250 defined but neither CONFIG_HPDCA nor CONFIG_HPAPCI defined, are you sure?
+#warning CONFIG_SERIAL_8250 defined but neither CONFIG_HPDCA nor CONFIG_HPAPCI defined, are you sure?
 #endif
 
 #ifdef CONFIG_HPAPCI