rtl8xxxu: Rename rtl8xxxu.c to rtl8xxxu_core.c

This renames the core file to rtl8xxxu_core.c in order to allow us to
keep the module nake rtl8xxxu.ko when refactoring the code into
multiple files.
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/realtek/rtl8xxxu/Makefile

 obj-$(CONFIG_RTL8XXXU)	+= rtl8xxxu.o
+
+rtl8xxxu-y	:= rtl8xxxu_core.o