net: sfp: return the number of written bytes for smbus single byte access

We expect the SFP write accessors to return the number of written bytes.
We fail to do so for single-byte smbus accesses, which may cause errors
when setting a module's high-power state and for some cotsworks modules.

Let's return the amount of written bytes, as expected.

Fixes: 7662abf ("net: phy: sfp: Add support for SMBus module access")
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20260105151840.144552-1-maxime.chevallier@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: minimaxwell

drivers/net/phy/sfp.c

@@ -765,7 +765,7 @@ static int sfp_smbus_byte_write(struct sfp *sfp, bool a2, u8 dev_addr,
 		dev_addr++;
 	}
 
-	return 0;
+	return data - (u8 *)buf;
 }
 
 static int sfp_i2c_configure(struct sfp *sfp, struct i2c_adapter *i2c)