MIPS: RB532: Remove obsolute reference to setup_serial_port

We are no longer using setup_serial_port. So just remove it
from the prom code.
Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/rb532/prom.c

@@ -37,8 +37,6 @@
 #include <asm/mach-rc32434/ddr.h>
 #include <asm/mach-rc32434/prom.h>
 
-extern void __init setup_serial_port(void);
-
 unsigned int idt_cpu_freq = 132000000;
 EXPORT_SYMBOL(idt_cpu_freq);