greybus: use PTR_ERR_OR_ZERO()

Coccinelle points out that the macro PTR_ERR_OR_ZERO() handles the
frequent case of converting a pointer into either error code (if its
value is an encoded error value) or 0 (otherwise).  Switch some code
in gb_power_supply_register() to use that macro.  I have verified
this is true of the kernel we're now working with (arche-6.0).
Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

drivers/staging/greybus/power_supply.c

@@ -547,10 +547,7 @@ static int gb_power_supply_register(struct gb_power_supply *gbpsy)
 
 	gbpsy->psy = power_supply_register(&connection->bundle->dev,
 					   &gbpsy->desc, &cfg);
-	if (IS_ERR(gbpsy->psy))
-		return PTR_ERR(gbpsy->psy);
-
-	return 0;
+	return PTR_ERR_OR_ZERO(gbpsy->psy);
 }
 #endif