microblaze: Fix variable types to remove W=1 warning

This patch removes this warning:
arch/microblaze/kernel/signal.c: In function 'setup_rt_frame':
arch/microblaze/kernel/signal.c:177:3: warning: signed and unsigned type
in conditional expression [-Wsign-compare]

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