power: generic-adc-battery: fix out-of-bounds write when copying channel properties

We did have sporadic problems in the pinctrl framework during boot
where a pin group name unexpectedly became NULL leading to a NULL
dereference in strcmp.

Detailled analysis of the failing cases did reveal that there were
two devm allocated objects close to each other. The second one was
the affected group_desc in pinmux and the first one was the
psy_desc->properties buffer of the gab driver.

Review of the gab code showed that the address calculation for
one memcpy() is wrong. It does

	properties + sizeof(type) * index

but C is defined to do the index multiplication already for
pointer + integer additions. Hence the factor was applied twice
and the memcpy() does write outside of the properties buffer.
Sometimes it happened to be the pinctrl and triggered the strcmp(NULL).

Anyways, it is overkill to use a memcpy() here instead of a simple
assignment, which is easier to read and has less risk for wrong
address calculations. So we change code to a simple assignment.

If we initialize the index to the first free location, we can even
remove the local variable 'properties'.

This bug seems to exist right from the beginning in 3.7-rc1 in

commit e60fea79 ("power: battery: Generic battery driver using IIO")

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Cc: stable@vger.kernel.org
Fixes: e60fea79 ("power: battery: Generic battery driver using IIO")
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>