cnss: Make bus bandwidth request optional

If device tree entry doesn't exist for bus bandwidth then
initialization of cnss driver fails. If device tree entry doesn't
exist then cnss driver needs to handle this case gracefully.
Modified driver to ignore the bus bandwidth request if device
tree entry doesn't exist.

Change-Id: Ib0e0d04d34d72bea633c74f436eeec625b986347
Signed-off-by: Prashanth Bhatta <bhattap@codeaurora.org>