USB: gadget: android: Integrate f_midi USB MIDI gadget driver part deux

free_ep_req and alloc_ep_req are static open coded and conflicting
in f_midi.c. The exported versions are present in f_sourcesink.c.
Changed names to protect the innocent.

Change-Id: I4aee40054b5715d0532d433d23dea2bccff7ec30
Signed-off-by: Mark Salyzyn <salyzyn@google.com>