usb: gadget: uac1: Fix redefiniction conflict for ac interface

Fix redefinition conflict for ac interface related descriptors' variable
names (ac_interface_desc and ac_header_desc) with f_midi.c and f_uac1.c
files by renaming same in f_uac1.c

Change-Id: Ie10dba1a9fad2cf76116adc162889e6339736c76
Signed-off-by: Mayank Rana <mrana@codeaurora.org>