mfd: lp8788: Fix an error handling path in lp8788_probe()

Should an error occurs in mfd_add_devices(), some resources need to be
released, as already done in the .remove() function.

Add an error handling path and a lp8788_irq_exit() call to undo a previous
lp8788_irq_init().

Fixes: eea6b7cc ("mfd: Add lp8788 mfd driver")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/18398722da9df9490722d853e4797350189ae79b.1659261275.git.christophe.jaillet@wanadoo.fr