serial: sh-sci: Use premultiplier to handle half sampling rate

On SCIx variants different from HSCIF, the bit rate is equal to the
sampling clock rate divided by half the sampling rate. Currently this is
handled by dividing the sampling rate by two, which was OK as it was
always even.

Replace halving the sampling rate by premultiplying the base clock
frequency by 2, to accommodate odd sampling rates on SCIFA/SCIFB later.

Replace the shift value in the BRG divider calculation by a
premultiplication of the base clock frequency too, for consistency.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>