ad525x_dpot: use correct rdac channel for ad5251/ad5252

The ad5251/ad5252 devices have rdac1 and rdac3, but no rdac0.  So make
sure we use the right channels so userspace gets correct data and not just
garbage.

Signed-off-by: steven miao <realmz6@gmail.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Cc: Michael Hennerich <michael.hennerich@analog.com>
Cc: Chris Verges <chrisv@cyberswitching.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>