power: supply: smb347-charger: Fix interrupt usage if interrupt is unavailable

The IRQ=0 could be a valid interrupt number in kernel because interrupt
numbers are virtual in a modern kernel. Hence fix the interrupt usage in
a case if interrupt is unavailable by not overriding the interrupt number
which is used by the driver.

Note that currently Nexus 7 is the only know device which uses SMB347
kernel diver and it has a properly working interrupt, hence this patch
doesn't fix any real problems, it's a minor cleanup/improvement.

Fixes: 99298de5 ("power: supply: smb347-charger: Replace mutex with IRQ disable/enable")
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>