staging: r8188eu: remove the sreset_priv structure

last_tx_time from struct sreset_priv is no longer used now that
last_tx_complete_time is gone.

This patch removes last_tx_time. This was the last remaining component of
struct sreset_priv, we can now remove the structure itself.

After removing include/rtw_sreset.h, it turned out that some definitions
in .c files were pulled in via rtw_sreset.h. Add the required include
files directly to make compilation succeed without rtw_sreset.h.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20211024180448.20624-6-martin@kaiser.cx
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>