msm_11ad: update ref_clk3 name to a more general name

The current assumption for the 11ad ref_clk to be rf_clk3 is wrong,
as in some platforms another clk source is used.
Update the name to a more generic one.

Change-Id: Ic91fafaec4490aeda0af626da4d4c27c42d681e4
Signed-off-by: Alexei Avshalom Lazar <ailizaro@codeaurora.org>