selftests: timers: clocksource-switch: fix passing errors from child

The return value from system() is a waitpid-style integer. Do not return
it directly because with the implicit masking in exit() it will always
return 0. Access it with appropriate macros to really pass on errors.

Fixes: 7290ce14 ("selftests/timers: Add clocksource-switch test from timetest suite")
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: John Stultz <jstultz@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>