clk: qcom: Check CLKFLAG_INIT_DONE flag for clk_get()

clk_get() should not return clock handle unless that clock has been
initialized. Add check for CLKFLAG_INIT_DONE to enforce this
requirement.

Change-Id: I201cf6988c2f5de8c92d1ab48024f1c6bd63b813
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>