i2c: mux: pca954x: call request irq after adding mux segments

The pca954x device do not have the ability to mask interrupts. For
i2c slave devices that also don't have masking ability (eg ltc1760
smbalert output) delay registering the irq until after the mux
segments have been configured. During the mux add_adaptor call the
core i2c system can register an smbalert handler which would then
be called immediately when the irq is registered. This smbalert
handler will then clear the pending irq.

This removes the need for the irq_mask / irq_unmask calls that were
original used to do this.

Signed-off-by: Phil Reid <preid@electromag.com.au>
Acked-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>