staging: wfx: drop unicode characters from strings

Smatch complains:

  hif_rx.c:235 hif_generic_indication() warn: format string contains non-ascii character '\xc2'
  hif_rx.c:235 hif_generic_indication() warn: format string contains non-ascii character '\xb0'
   234                  if (!wfx_api_older_than(wdev, 1, 4))
   235                          dev_info(wdev->dev, "Rx test ongoing. Temperature: %d°C\n",
                                                                                     ^
   236                                   body->data.rx_stats.current_temp);

So, replace the unicode character.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20201009171307.864608-8-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>