mfd: arizona: Remove duplicate set of ret variable

arizona_poll_reg already returns ETIMEDOUT if we don't see the expected
register changes before the time out, so remove pointless local setting of
ETIMEDOUT.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>