f_rmnet: Don't register for already registerd SMD platform driver

currently, USB driver is registering for platform driver that is
already registered. Avoid this multiple registrations.

CRs-fixed: 403650
Change-Id: I4446f00a5634e4c1146b84381da063284aeed6c9
Signed-off-by: Chiranjeevi Velempati <cvelempa@codeaurora.org>