rtl8xxxu: Use REG_EFUSE_TEST register only on multifunctional devices

rtl8192cu driver doesn't read/write the REG_EFUSE_TEST register.
Neither does the rtl8188eu driver.  Do it only for multifunc devices
RTL8723AU/RTL8723BU.

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>