Fix for Issue #7308 (Deep_sleep_lock Ticker.h Issue)

[mprse]

Description

Provide fix for issue #7308.

Code used to reproduce the issue:

void test()
{
    Ticker ticker;

    ticker.attach_us(callback(stop_gtimer_set_flag), 1000);
    while (!ticker_callback_flag);
    ticker.detach();
}

Test has been built with MBED_SLEEP_TRACING_ENABLED flag. Usage of Ticker should result in locking deep sleep.

Results on master without fix:

[1548322380.64][CONN][INF] found KV pair in stream: {{__testcase_name;Test}}, queued...
[1548322380.71][CONN][RXD] >>> Running case #1: 'Test'...
[1548322380.78][CONN][INF] found KV pair in stream: {{__testcase_start;Test}}, queued...
-> [1548322380.83][CONN][RXD] LOCK: Ticker.h, ln: 125, lock count: 1
-> [1548322380.89][CONN][RXD] Unlocking sleep for driver that was not previously locked: Ticker.cpp, ln: 32
[1548322380.97][CONN][INF] found KV pair in stream: {{__testcase_finish;Test;1;0}}, queued...

Results with this fix:

[1548322463.11][CONN][INF] found KV pair in stream: {{__testcase_name;Test}}, queued...
[1548322463.18][CONN][RXD] >>> Running case #1: 'Test'...
[1548322463.24][CONN][INF] found KV pair in stream: {{__testcase_start;Test}}, queued...
-> [1548322463.28][CONN][RXD] LOCK: Ticker.cpp, ln: 61, lock count: 1
-> [1548322463.32][CONN][RXD] UNLOCK: Ticker.cpp, ln: 32, lock count: 0
[1548322463.39][CONN][INF] found KV pair in stream: {{__testcase_finish;Test;1;0}}, queued...

Pull request type

[X] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

[ciarmcom]

@mprse, thank you for your changes.
@ARMmbed/mbed-os-core @ARMmbed/mbed-os-maintainers please review.

[deepikabhavnani]

Looks good :+

[cmonr]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 12 of 12 test jobs passed
Build number : 1
Build artifacts