clk: qcom: clock-pll: Conditionally fail round_rate if a pll is prepared

Some clock trees allow ping-ponging between PLLs in
order to optimize frequency switching. To allow
the mux round_rate logic to work, fail round_rate
if a PLL is already prepared, if a feature flag is
set.

Change-Id: I63bcc3bcc9967348b6986b47de529ccbfa78ad32
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>