staging: et131x: Remove unecessary *etdev NULL check in et1310_phy_init()

It is impossible to get to this function with the pointer being NULL.
Tested on an ET-131x device.

Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>