usb: gadget: f_hid: use free_ep_req()

We should always use free_ep_req() when allocating requests with
alloc_ep_req().

Change-Id: Ifa23bf08baf00eb6a1516726de7655901a532cdc
Signed-off-by: Felipe F. Tonello <eu@felipetonello.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Git-commit: 14794d7133d0f16b4901207a489f04e4e700166a
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git


Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>