power: supply: bq25890_charger: fix incorrect error return when bq25890_field_read fails

Currently a read failure by bq25890_field_read on F_DEV_REV is returning
an error in id instead of rev. Fix this by returning the value in rev.

Addresses-Coverity: ("Copy-paste error")
Fixes: d20267c9 ("power: supply: bq25890_charger: Add support of BQ25892 and BQ25896 chips")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>