fwget: fix installing multiple firmware files

Remove quotes around the list of packages to install.
Otherwise pkg is only fed one argument which results in a
non-working solution:
pkg: No packages available to install matching 'wifi-firmware-ath10k-kmod wifi-firmware-mt76-kmod wifi-firmware-ath11k-kmod' have been found in the repositories

MFC after: 10 days
Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D41474