USB: gadget: functionfs: stale Makefile entry removed

Removed entry referencing g_eth_ffs.c file from Makefile.
The file never existed and the line was a leftover from a
developing process.

Signed-off-by: Michal Nazarewicz <m.nazarewicz@samsung.com>
Reported-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>