rtl8xxxu: Handle XTAL_K value in efuse specific location

Retrieve the XTAL_K value in the parse_efuse() functions as it's
location various on a per device basis. For parts that do not provide
an XTAL_K value, skip setting it.

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