Properly check the return code of ffs_func_revmap_intf() and
ffs_func_revmap_ep() for a non-negative value.

Instead of checking the return code, the comparison was performed for the last
parameter of the function calls, because of wrong parenthesis.

This also fixes the following static checker warning:
drivers/usb/gadget/function/f_fs.c:3152 ffs_func_req_match()
warn: always true condition '(((creq->wIndex)) >= 0) => (0-u16max >= 0)'
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felix Hädicke <felixhaedicke@web.de>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>