i2c: xiic: Switch from waitqueue to completion

There will never be threads queueing up in the xiic_xmit(), use
completion synchronization primitive to wait for the interrupt
handler thread to complete instead as it is much better fit and
there is no need to overload it for this purpose.

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>