drivers/nble: Use semaphore to control notification rate

At the moment firmware reads everything we send to it over UART and
when an application tries to send too much notifications we are stuck.
The proper approach would be forcing firmware to read from UART only
when there is available memory. In this case we would block on UART
FIFO and everything handles gracefully.
This workaround mimics solution used by firmware project by allowing
to send next notification only after previous was sent correctly.

Change-Id: I56dd5abc4d13735d285415da04db75e7bb8bc459
Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>