staging: r8188eu: read timestamp from ieee80211_mgmt

Read the 64-bit timestamp from struct ieee80211_mgmt instead of parsing
the beacon message ourselves.

Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20221126160129.178697-3-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>