microblaze: Use unsigned type for limit comparison in cache.c

The patch removes warnings:
arch/microblaze/kernel/cpu/cache.c:146:14: warning: comparison of
unsigned expression < 0 is always false [-Wtype-limits]

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

diff --cc arch/microblaze/kernel/cpu/cache.c
Simple merge