rtl8xxxu: Set the correct thermal meter register for 8723bu

Older chips use RF register 0x24 to set the thermal meter. Newer chips
use register 0x42.

This change makes sure to set the correct thermal meter register
depending on the chip.

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