staging: r8188eu: pass a struct recv_buf to rtw_read_port

The rtw_read_port function needs a struct recv_buf for preparing the usb
transfer.

Replace its u8 *rmem parameter with a struct recv_buf pointer to avoid
casts in the caller and in rtw_read_port.

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/20230206201800.139195-2-martin@kaiser.cx


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