USB: ehci-msm2: Use clk_round_rate to get MAX clock frequency

USB core can not tolerate any frequency change at runtime due
to hardware limitation. Hence it must be set to Max supported
clk frequency.

Instead of setting rate as INT_MAX, use standard clock driver
API - clk_round_rate to obtain the maximum supported rate for
the clock.

Change-Id: I12873076a43efe64a51e1cdfa279eb343e6a9947
Signed-off-by: Chandra Devireddy <cdevired@codeaurora.org>