drivers/misc/ad525x_dpot-i2c.c: add i2c support for AD5161

Commit 6c536e4c ("ad525x_dpot: add support for SPI parts") added
support for the AD5161 through SPI, but the device supports both I2C and
SPI (depending on the DIS pin), so add it to -i2c as well.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>