ath11k-firmware: move Download eval to top level

ath11k-firmware: move Download eval to top level

Move the $(eval $(call Download,ath11k-legacy-firmware)) from inside
Build/Prepare to the top level, right after the Download definition.

When the eval is inside Build/Prepare, it only runs at recipe execution
time (during compilation). This makes the legacy firmware source
invisible to 'make download', which only evaluates Download definitions
at Makefile parse time. As a result, offline builds fail because the
source was never fetched during the download phase.

This is consistent with how ath10k-ct-firmware handles multiple
Download definitions (all at top level).

Signed-off-by: Michael Pfeifroth <michael.pfeifroth@westermo.com>
Link: https://github.com/openwrt/openwrt/pull/23628
Signed-off-by: Robert Marko <robimarko@gmail.com>