drivers: serial: telink_b91: fix uart unknown state after boot from mcuboot

mcuboot may use a uart as logger. After application boot from mcuboot,
uart remains in some unknown state. It could leads to some unpredictable
uart behaviour. This commit fixes the issue by resetting of uart state
at the beginning of uart init.

Signed-off-by: Alex Kolosov <rikorsev@gmail.com>