rtl8xxxu: Init H2C command register for 8723bu

In addition make register read/write flow match closer to vendor
driver flow. This is mainly to be able to compare the register write
log with the vendor driver, and can be optimized later once 8723bu
support is working.

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