MIPS: Alchemy: Fix warnings in DB1x00 / PB1000 / PB1550 board setup code

This patch fixes warnings due to potentially unused
variables in board setup code or mixed variables
declaration and code (forbidden by ISO C90).

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>