wifi: cfg80211: Add KHZ_PER_GHZ to units.h and reuse

The KHZ_PER_GHZ might be used by others (with the name aligned
with similar constants). Define it in units.h and convert
wireless to use it.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://msgid.link/20240215154136.630029-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Johannes Berg <johannes.berg@intel.com>