atm: uPD98402: fix incorrect allocation

dev->dev_data is set in zatm.c, calling zatm_start() will overwrite this
dev->dev_data in uPD98402_start() and a subsequent PRIV(dev)->lock
(i.e dev->phy_data->lock) will result in a null-ptr-dereference.

I believe this is a typo and what it actually want to do is to allocate
phy_data instead of dev_data.

Signed-off-by: Tong Zhang <ztong0001@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>