wifi-scripts: fix failing mesh setup with missing wpa_supplicant

wifi-scripts: fix failing mesh setup with missing wpa_supplicant

The initialization of mesh interfaces currently fail when wpa_supplicant
is not installed. This is due to the script calling the wpa_supplicant
feature indicator without verifying wpa_supplicant is installed at all.

To avoid failing, first check if wpa_supplicant is installed before
determining the available featureset.

Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit 1be18c6daad83bc4198dc7aefd9979b7fe8fbfd4)