iio:ad5064: Make sure ad5064_i2c_write() returns 0 on success

iio:ad5064: Make sure ad5064_i2c_write() returns 0 on success

i2c_master_send() returns the number of bytes transferred on success while
the ad5064 driver expects that the write() callback returns 0 on success.
Fix that by translating any non negative return value of i2c_master_send()
to 0.

Fixes: commit 6a17a0768f77 ("iio:dac:ad5064: Add support for the ad5629r and ad5669r")
Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>