rtl8xxxu: move rtl8723b related code into rtl8xxxu_8723b.c

This moves the rtl8723b code into it's own file. This is purely a code
moving exercise, no functional changes. This did expose
rtl723a_h2c_cmd() as a function that should be refactored into a gen1
and a gen2 version.

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