microblaze: Use unsigned type for "for" loop because of comparison-kgdb.c

This patch removes warnings reported by W=1:
arch/microblaze/kernel/kgdb.c: In function 'pt_regs_to_gdb_regs':
arch/microblaze/kernel/kgdb.c:43:16: warning: comparison between signed
and unsigned integer expressions [-Wsign-compare]
arch/microblaze/kernel/kgdb.c:51:16: warning: comparison between signed
and unsigned integer expressions [-Wsign-compare]
arch/microblaze/kernel/kgdb.c: In function 'gdb_regs_to_pt_regs':
arch/microblaze/kernel/kgdb.c:77:16: warning: comparison between signed
and unsigned integer expressions [-Wsign-compare]
arch/microblaze/kernel/kgdb.c: In function
'sleeping_thread_to_gdb_regs':
arch/microblaze/kernel/kgdb.c:99:16: warning: comparison between signed
and unsigned integer expressions [-Wsign-compare]
arch/microblaze/kernel/kgdb.c:103:16: warning: comparison between signed
and unsigned integer expressions [-Wsign-compare]

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