staging: axis-fifo: add CONFIG_OF dependency

staging: axis-fifo: add CONFIG_OF dependency

When building without CONFIG_OF, the compiler loses track of the flow
control in axis_fifo_probe(), and thinks that many variables are used
without an initialization even though we actually leave the function
before the first use:

drivers/staging/axis-fifo/axis-fifo.c: In function 'axis_fifo_probe':
drivers/staging/axis-fifo/axis-fifo.c:900:5: error: 'rxd_tdata_width' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  if (rxd_tdata_width != 32) {
     ^
drivers/staging/axis-fifo/axis-fifo.c:907:5: error: 'txd_tdata_width' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  if (txd_tdata_width != 32) {
     ^
drivers/staging/axis-fifo/axis-fifo.c:914:5: error: 'has_tdest' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  if (has_tdest) {
     ^
drivers/staging/axis-fifo/axis-fifo.c:919:5: error: 'has_tid' may be used uninitialized in this function [-Werror=maybe-uninitialized]

When CONFIG_OF is set, this does not happen, and since the driver cannot
work without it, just add that option as a Kconfig dependency.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>