drivers: uart: stm32: fix handling interrupt and async api in isr

When using the uart driver with interrupt and async api
at the same time (instance for interrupt and instance for async),
the transmission complete interrupt was handled in the async
handling section, even when interrupt driven api is used.
This caused transmission to not work properly in interrupt mode.
The fix is to move the interrupt mode handling to the begginning
of the isr. If async mode is used then interrupt mode code
will not be run.

Signed-off-by: Shlomi Vaknin <shlomi.39sd@gmail.com>