drivers: serial: uart_sam0: Fix async tx done event triggering to early.

uart_sam0_dma_tx_done callback triggers when the last byte
is transferred from the tx sram buffer to the sercom DATA register.
However the byte has yet to be transmitted completely which can lead to
incorrect event handling if UART_TX_DONE is expected to signal
the end of transmission.

Signed-off-by: Ron Smith <rockyowl171@gmail.com>