usb: gadget: u_bam: Use already initialized BAM connection indexes

Function drivers already initialize src/dst_connection_idx for
BAM2BAM transports, but, u_bam still queries the same again
with port number hardcoded to '0'.
This works fine if only instance of a function is used. In case
multiple rmnet instances are used then u_bam ends up using same
BAM pipe with both RMNETs when setting up endless DBM transfers.

Change-Id: Ice98f3bd2d91e26d2efecbaf3541787117e90027
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>