i2c: xiic: move the xiic_process to thread context

The xiic_process is a 154 line code that runs in isr context currently
move it to thread context. Also the name xiic_process suggests that the
intension was to run in process context.

Signed-off-by: Shubhrajyoti Datta <shubhraj@xilinx.com>
[wsa: initialized irqreturn_t to IRQ_NONE instead of IRQ_HANDLED]
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>