staging: rtl8192u: do not use undefined $(TOPDIR) in Makefile

drivers/staging/rtl8192u/ieee80211/Makefile uses $(TOPDIR) to configure
an include directory, without defining this variable first. The path
which is configured is therefore "/drivers/net/wireless", which does not
seem to be the intended path.

Remove the offending line.

Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>