staging: rtl8192e: Replace macro skb_tail_pointer_rsl with standard function

Replace macro skb_tail_pointer_rsl with function skb_tail_pointer to
increase readablility.

Signed-off-by: Philipp Hortmann <philipp.g.hortmann@gmail.com>
Link: https://lore.kernel.org/r/6885112850b48c0f9ff16655f7d9efe085ccc001.1679504314.git.philipp.g.hortmann@gmail.com


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