hso: rename hso_dev into serial in hso_free_interface()

In other functions of the driver, variables of type "struct hso_serial"
are denoted by "serial" and variables of type "struct hso_device" are
denoted by "hso_dev". This patch makes the hso_free_interface()
consistent with these notations.

Signed-off-by: Olivier Sobrie <olivier@sobrie.be>
Signed-off-by: David S. Miller <davem@davemloft.net>