msm:ipa: Return mux id only for valid interface

Value of MAX_NUM_OF_MUX_CHANNEL is greater than
number of valid interfaces. So empty interface
is also getting mux id. Return mux id only for valid
interfaces.

Change-Id: I7852df0aa0ccee781c1bf6857a4183b99194f3ee
Acked-by: Pooja Kumari <kumarip@qti.qualcomm.com>
Signed-off-by: Mohammed Javid <mjavid@codeaurora.org>