rtl8xxxu: Split rtl8723a_h2c_cmd() into a gen1 and a gen2 version

The H2C API is completely different between gen1 and gen2 parts, so
there is little point trying to treat this as a generic function. All
calls to *_h2c_cmd() will always come from a gen1 or a gen2 specific
function.

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