microblaze: bootm: Fix compiler warning

Fix build warning in arch/microblaze/lib/bootm.c with gcc 7.3.0:

  warning: this 'if' clause does not guard... [-Wmisleading-indentation]

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>