firmware-samsung-pxa1908: split downstream wifi-enablement to subpackage (MR 5295)

Resolves this harmless, but annoying error on mainline:

 * Enabling wlan driver ...
/lib/rc/sh/openrc-run.sh: line 14: can't create /sys/devices/platform/sd8x-rfkill/pwr_ctrl: nonexistent directory
 * ERROR: sd8x-wlan failed to start
[ci:skip-build]: already built successfully in CI