usb: g_ffs: fix compilation warning

usb: g_ffs: fix compilation warning

If USB_FUNCTIONFS is selected without USB_FUNCTIONFS_ETH and
USB_FUNCTIONFS_RNIS, u_ether.h won't be included and then
USB_ETHERNET_MODULE_PARAMAETERS macro won't be available causing the
following warning compilation:

drivers/usb/gadget/g_ffs.c:81:1: warning: data definition has no type or
storage class [enabled by default]
drivers/usb/gadget/g_ffs.c:81:1: warning: type defaults to ‘int’ in
declaration of ‘USB_ETHERNET_MODULE_PARAMETERS’ [-Wimplicit-int]
drivers/usb/gadget/g_ffs.c:81:1: warning: function declaration isn’t a
prototype [-Wstrict-prototypes]

This patch fixes the warning by making USB_ETHERNET_MODULE_PARAMETERS to
be used iff u_ether.h is included, otherwise it is not needed.

Signed-off-by: David Cohen <david.a.cohen@linux.intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>