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>