i2c: xiic: Defer xiic_wakeup() and __xiic_start_xfer() in xiic_process()

The __xiic_start_xfer() manipulates the interrupt flags, xiic_wakeup()
may result in return from xiic_xfer() early. Defer both to the end of
the xiic_process() interrupt thread, so that they are executed after
all the other interrupt bits handling completed and once it completely
safe to perform changes to the interrupt bits in the hardware.

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