drivers: xmc4xxx_uart: Delay transmit interrupt until byte is sent out

Generate the Tx service request after the symbol is shifted out of the
UART. This is useful when the UART is connected to an RS485 transducer
which has a separate transmit enable gpio/line. Hence it's important
to know when the transmission actually finishes so that the drive
enable line can be disabled.

Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>