usb: gadget: u_bam_data: Fix port parameter in start/stop_rndis_ipa()

commit 9a03014e (usb: gadget: allow working with multiple u_bam_data
ports simultaneously) changed the mapping of the RNDIS entry in
the global bam2bam_data_ports array but missed updating these two
functions, so they incorrectly use index 0. Fix these to use the
u_bam_data_func_to_port() helper.

Change-Id: Ib4a44b26a473e2021d748486820d33541b8e1bae
Signed-off-by: Jack Pham <jackp@codeaurora.org>