apps/iperf: add iperf_api semaphore on reporter callback

apps/iperf: add iperf_api semaphore on reporter callback

when iperf bi-directional test, freelist array value has been corrupted
due to simultaneous access on it.
 - freelist array is used on __dtoa function called by snprintf
 - it causes data abort during a long-run test
iperf reporter callback function is called by timer interrupt so,
to guarantee completion of it we've added semaphore on reporter callback
function.

Change-Id: Ib0549de6c850008eb825a6d484205c0b84725b6d
Signed-off-by: Jin-Seong Kim <jseong82.kim@samsung.com>
Signed-off-by: EunBong Song <eunb.song@samsung.com>