msm: dba: define null function to register client

All clients need to call register API exposed by DBA module to check
if the down stream transmitter driver is configured or not. In case
DBA is not enabled, define a null register function which returns
a NULL pointer to let the clients know of non availability of DBA.

Change-Id: I2b6066babe5a7e464a31752720da8f299997e977
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>
Signed-off-by: Siddharth Zaveri <szaveri@codeaurora.org>