staging: r8188eu: remove unused da parameter

All callers of issue_probereq_p2p set the da parameter to NULL. Remove
this parameters and the code that runs only for da != NULL.

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/20221202092525.403887-3-martin@kaiser.cx
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>