can: gs_usb: gs_usb_set_phys_id(): return with error if identify is not supported

can: gs_usb: gs_usb_set_phys_id(): return with error if identify is not supported

Until commit 409c188c57cd ("can: tree-wide: advertise software
timestamping capabilities") the ethtool_ops was only assigned for
devices which support the GS_CAN_FEATURE_IDENTIFY feature. That commit
assigns ethtool_ops unconditionally.

This results on controllers without GS_CAN_FEATURE_IDENTIFY support
for the following ethtool error:

| $ ethtool -p can0 1
| Cannot identify NIC: Broken pipe

Restore the correct error value by checking for
GS_CAN_FEATURE_IDENTIFY in the gs_usb_set_phys_id() function.

| $ ethtool -p can0 1
| Cannot identify NIC: Operation not supported

While there use the variable "netdev" for the "struct net_device"
pointer and "dev" for the "struct gs_can" pointer as in the rest of
the driver.

Fixes: 409c188c57cd ("can: tree-wide: advertise software timestamping capabilities")
Link: http://lore.kernel.org/all/20220818143853.2671854-1-mkl@pengutronix.de
Cc: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>