staging: r8188eu: remove unused fields in struct recv_buf

The fields len, ref_cnt and transfer_len in struct recv_buf are set
but not used. Remove them.

Signed-off-by: Abdun Nihaal <abdun.nihaal@gmail.com>
Link: https://lore.kernel.org/r/fe5565c02a01babf64469333dae73cbdd841d393.1646321515.git.abdun.nihaal@gmail.com


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