i2c: i2c-nct6775: fix -Wimplicit-fallthrough

Depending on a compiler, `case I2C_SMBUS_BYTE_DATA` can trigger
-Wimplicit-fallthrough / unannotated fallthrough as it doesn't
mention it anyhow.
Add a `fallthrough` attribute to silence this.

Fixes: 480fefb ("i2c: busses: Add SMBus capability to work with OpenRGB driver control")
Signed-off-by: Alexander Lobakin <alobakin@pm.me>

drivers/i2c/busses/i2c-nct6775.c

@@ -219,6 +219,7 @@ static s32 nct6775_access(struct i2c_adapter * adap, u16 addr,
 			break;
 		case I2C_SMBUS_BYTE_DATA:
 			tmp_data.byte = data->byte;
+			fallthrough;
 		case I2C_SMBUS_BYTE:
 			outb_p((addr << 1) | read_write,
 			       SMBHSTADD);