hdlcdrv: replace unnecessary assertion in hdlcdrv_register

In hdlcdrv_register, failure to register the driver causes a crash.
The three callers of hdlcdrv_register all pass valid pointers and
do not fail. The patch eliminates the unnecessary BUG_ON assertion.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>