CHROMIUM: mesh: fix backtrace in airtime_link_metric_get() function

Cherry picked form Eureka,
commit <8d7ee0b45a01bf90241c545f5b69e3eb8529d247> - "fix backtrace in
airtime_link_metric_get() function"

BUG=b:134397720
TEST=mesh connectivity
Change-Id: Ic0d43e0491914a9a8f23e4e104a59bfa64d62aa4
Signed-off-by: Ratheesh S P <ratheeshs@google.com>
Reviewed-on: https://chromium-review.googlesource.com/1642693


Commit-Ready: Ratheesh Selvakumar Pappa <ratheeshs@google.com>
Tested-by: Ratheesh Selvakumar Pappa <ratheeshs@google.com>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Xiaodong Sun <xdsun@google.com>
Reviewed-by: Julan Hsu <julanhsu@google.com>