i2c: i801: add helper i801_set_hstadd()

Factor out setting SMBHSTADD to a helper. The current code makes the
assumption that constant I2C_SMBUS_READ has bit 0 set, that's not ideal.
Therefore let the new helper explicitly check for I2C_SMBUS_READ.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Wolfram Sang <wsa@kernel.org>