i2c: mv64xxx: don't override deferred probing when getting irq

There is no reason to use platform_get_irq() for non-DT probing and
irq_of_parse_and_map() for DT probing. Indeed, platform_get_irq()
works fine for both.

In addition, using platform_get_irq() properly returns -EPROBE_DEFER
when the interrupt controller is not yet available, so instead of
inventing our own error code (-ENXIO), return the one provided by
platform_get_irq().

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>