staging: rtl8712: fix use after free bugs

_Read/Write_MACREG callbacks are NULL so the read/write_macreg_hdl()
functions don't do anything except free the "pcmd" pointer.  It
results in a use after free.  Delete them.

Fixes: 2865d42c78a9 ("staging: r8712u: Add the new driver to the mainline kernel")
Cc: stable <stable@kernel.org>
Reported-by: Zheng Wang <hackerzheng666@gmail.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/Yw4ASqkYcUhUfoY2@kili
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
