rtl8xxxu: Fix error handling if rtl8xxxu_init_device() fails

For some reason we lost the code bailing if rtl8xxxu_init_device()
returned an error.

This catches the error and also cleans up the error handling.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>