tools: testing: selftests: mq_perf_tests: Fix infinite loop on ARM

We can't use a char type to check for a negative return value since char
isn't guaranteed to be signed. Indeed, the char type tends to be unsigned on
ARM.

Signed-off-by: dann frazier <dann.frazier@canonical.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>