i2c: cadence: Remove `irq` field from driver state struct

The irq field of the driver state struct is only used in the probe
function. There is no need to keep it around. Remove it from the state
struct and use a on-stack variable instead.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Reviewed-by: Michal Simek <michal.simek@amd.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>