smb347-charger: Move IRQ enabling to the end of probe

There is a potential problem if we call smb347_irq_enable() from
smb347_irq_init() because smb347_irq_enable() makes the device registers
read-only once it returns and smb347_irq_init() expects them to still be
read-write. Currently no harm happens because it is the last call we make
in smb347_irq_init().

Anyway a better place for enabling IRQs is at the end of probe function
and this is also symmetric to call smb347_irq_disable() which is done at
the beginning of remove function.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>