usb: dwc3: gadget: Refactor dwc3_gadget_ep_dequeue

The flow from function dwc3_gadget_ep_dequeue() is not easy to follow.
Refactor it for easier read. No functional change in this commit.

Change-Id: I254081abaf3ff7c6a7a598bf859804ecf3454a15
Signed-off-by: Thinh Nguyen <thinhn@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>
Git-commit: fcd2def6639293c2bde2dc4f5dab63641a60b5b3
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[mrana@codeaurora.org: resolve merge conflict related to debug log]
Signed-off-by: Mayank Rana <mrana@codeaurora.org>