driver: uart: stm32: Add workaround for DMAT errata in low-power modes

This change adds additional workaround for following errata:
"USART does not generate DMA requests after setting/clearing DMAT bit"
Instead of keepint DMAT bit set, it sends first byte by polling
in firmware. This prevents additional power consumption in STOP mode,
caused by keeping DMAT bit set.

Signed-off-by: Adam Berlinger <adam.berlinger@st.com>