staging: rtl8192u: remove unneeded compiler flags

-std=gnu89 is specified by the top Makefile. Adding it in a driver
Makefile is redundant.

A driver should avoid specifying the optimization flag.
-O2, -O3, or -Os is passed by the top Makefile based on the
CONFIG_CC_OPTIMIZE_FOR_* option.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20200104162136.19170-3-masahiroy@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>