staging: r8188eu: remove unused field ptail

The field ptail in struct recv_buf is set but not used.
Remove it.

Signed-off-by: Abdun Nihaal <abdun.nihaal@gmail.com>
Link: https://lore.kernel.org/r/9574ad2dfba0cc444ce1dea1268661ed32b84799.1646321515.git.abdun.nihaal@gmail.com


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