fix(st-uart): correctly check UART enabled in flush fonction

Use tst instead of ands to check USART_CR1_UE bit is set. If not exit
the flush function.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: Ibd2e18f6d8837073d0bbcb150e993985d3c0dd6f
1 file changed