commit f7bc126 (Re-work Rtl8195AM ticker) tried to address ticker drifting
issue, but in fact broke ticker functionality. This patch adjusts us_ticker
porting to unbreak it, and to fix the following tests:

mbed-os-tests-mbed_drivers-ticker
mbed-os-tests-mbed_drivers-timeout
mbed-os-tests-mbedmicro-rtos-mbed-isr
mbed-os-features-frameworks-utest-tests-unit_tests-minimal_async_scheduler
mbed-os-features-frameworks-utest-tests-unit_tests-case_control_async
mbed-os-features-frameworks-utest-tests-unit_tests-case_async_validate

Changes are:

1. Dont disable timer after every us_ticker interrupt. That basically
   breaks ticker functionality.

2. Fine-tune us to tick conversion macro. Previous conversion method
   yields 1 tick drift every 2 ms.

3. Remove special placement of ticker objects. No longer necessary.

Signed-off-by: Tony Wu <tonywu@realtek.com>