Makefile assumes kernel already has RTL8XXXU enabled

[brentr]

First line of Makefile is:
obj-$(CONFIG_RTL8XXXU) += rtwifi.o
However, usually the kernel we're building against does not (yet) have RTL8XXXU support.
Resulting in nothing being built.

Fix is to change that first line to:
obj-m += rtwifi.o