staging: r8188eu: remove some obsolete comments

Remove obsolete comments in validate_recv_data_frame.
There is no prxcmd variable (any more?).

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Link: https://lore.kernel.org/r/20221106124901.720785-3-martin@kaiser.cx


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