regulator: ltc3589: Make IRQ optional

It's perfectly valid to use the LTC3589 without an interrupt pin
connected to it. Currently, the driver probing fails when client->irq
is 0 (which means "no interrupt"). Don't register the interrupt
handler in that case but successfully finish the device probing instead.

Signed-off-by: Bernhard Walle <bernhard@bwalle.de>
Signed-off-by: Mark Brown <broonie@kernel.org>