usb: gadget: f_fs: Fix compile warnings

Fix the following compiler warning(s) from gcc 6.1.1 with -Werror enabled:

 drivers/usb/gadget/function/f_fs.c:1066:
   format '%d' expects argument of type 'int', but argument 4 has
	type 'ssize_t {aka long int}' [-Werror=format=]
   ffs_log("queued %d bytes on %s", data_len, epfile->name);

 drivers/usb/gadget/function/f_fs.c:1110:
   format '%d' expects argument of type 'int', but argument 4 has
	type 'ssize_t {aka long int}' [-Werror=format=]
   ffs_log("queued %d bytes on %s", data_len, epfile->name);

Fixes: cb9bad16 ("usb: gadget: f_fs: Add support for ipc logging")
Change-Id: Ic0dedbada94266af53ba3824954c7271ec0453b3
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>