drivers: uart_xmc4xxx: Change order of functions

This patch is in preparation of uart async support. There are no
functional changes. The patch changes the ordering of functions to
organize shared functions between async/interrupt driven in the same order.
Also move uart_xmc4xxx_init() so that a forward declaration can be
removed.

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