rtc: rv3028: fix clock output support

rv3028_clkout_set_rate unconditionally sets RV3028_CLKOUT_CLKOE but
clk_set_rate may be called with the clock disabled. Ensure the clock is
kept disabled if it was not yet enabled.

Also, the actual rate was overwritten when enabling the clock, properly
write to the register only once.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201009153101.721149-1-alexandre.belloni@bootlin.com