USB: f_rmnet: Check for ep desc rather than driver data

Function drivers should not check the driver_data to configure
the endpoints in set_alt, as the endpoint is fixed and is assigned
as a part of bind. Instead, should check if the descriptor is NULL
or not and based on that should select the endpoint configuration,
as endpoint descriptor is set to NULL on every function disable, which
gets called on cable disconnect, USB Reset or configuration change.

CRs-Fixed: 359471
Change-Id: I14a483805e515868642ed879ee3f38846c7a7a47
Signed-off-by: Chiranjeevi Velempati <cvelempa@codeaurora.org>