microblaze: Use unsigned type for proper comparison in cpuinfo*.c

Compare the same types together.

Compilation warnings:
arch/microblaze/kernel/cpu/cpuinfo-pvr-full.c: In function
'set_cpuinfo_pvr_full':
arch/microblaze/kernel/cpu/cpuinfo-pvr-full.c:47:20: warning: comparison
between signed and unsigned integer expressions [-Wsign-compare]
arch/microblaze/kernel/cpu/cpuinfo-pvr-full.c:52:19: warning: comparison
between signed and unsigned integer expressions [-Wsign-compare]
arch/microblaze/kernel/cpu/cpuinfo-pvr-full.c:57:18: warning: comparison
between signed and unsigned integer expressions [-Wsign-compare]
arch/microblaze/kernel/cpu/cpuinfo-pvr-full.c:94:20: warning: comparison
between signed and unsigned integer expressions [-Wsign-compare]
arch/microblaze/kernel/cpu/cpuinfo-static.c: In function
'set_cpuinfo_static':
arch/microblaze/kernel/cpu/cpuinfo-static.c:40:20: warning: comparison
between signed and unsigned integer expressions [-Wsign-compare]

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