arm64: mte: style: Simplify bool comparison

Fix the following coccicheck warning:
./tools/testing/selftests/arm64/mte/check_buffer_fill.c:84:12-35:
WARNING: Comparison to bool

Signed-off-by: YANG LI <abaci-bugfix@linux.alibaba.com>
Reported-by: Abaci <Robot&lt;abaci@linux.alibaba.com>
Link: https://lore.kernel.org/r/1610357737-68678-1-git-send-email-abaci-bugfix@linux.alibaba.com
Signed-off-by: Will Deacon <will@kernel.org>