rtl8xxxu: Skip disabled efuse words early

Avoid a negative conditional and an extra level of indentation in the
bigger part of the loop body.

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