i2c: xiic: Drop broken interrupt handler

The interrupt handler is missing locking when reading out registers
and is racing with other threads which might access the driver. Drop
it altogether, so that the threaded interrupt is always executed, as
that one is already serialized by the driver mutex. This also allows
dropping local_irq_save()/local_irq_restore() in xiic_start_recv().

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