fff-wireless: fix shellcheck errors

In src/packages/fff/fff-wireless/files/lib/functions/fff/wireless line 26:
    local radio="radio$(echo $phy | tr -d -C [0-9])"
                                 ^-- SC2086: Double quote to prevent globbing and word splitting.
                                                 ^-- SC2060: Quote parameters to tr to prevent glob expansion.

Signed-off-by: Tim Niemeyer <tim@tn-x.org>
Reviewed-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Tested-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>