regulator: ad5398: Fix return value of ad5398_write_reg

i2c_master_send() returns the number of bytes written on success.
So current code returns 2 if ad5398_write_reg() success.
This return value is propagated to .set_current_limit, .enable and .disable
callbacks of regulator_ops. This can be a problem, for example, if the
users test if the return value of regulator_set_current_limit() is 0.
Fix it by making ad5398_write_reg() return 0 on success.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mark Brown <broonie@kernel.org>