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>
Git-commit: ae2019d83ecf14454315240c16dc3136212f0da2
Git-repo: https://android.googlesource.com/kernel/common/


[mrana@codeaurora.org: already alloc_ep_req renamed, merge conflict]
Signed-off-by: Mayank Rana <mrana@codeaurora.org>