phy: safer to_phy() macro

This makes to_phy() macro work with other variable names
besides "dev".

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Tested-by: Vivek Gautam <gautam.vivek@samsung.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>