USB: ehci-msm2: Replace clk_enable/disable with clk_prepare_xx APIs

clk_prepare and clk_unprepare APIs can be called from non-atomic
context only. Where as clk_enable and clk_disable APIs can be called
from atomic context.
To enable a clock, a driver should call both clk_prepare and
clk_enable. Similarly for disabling a clock - clk_disable and
clk_unprepare should be called.

Change-Id: Ic5cf1c1cc60df9d4e510c719fe6b75d317de2f2f
Signed-off-by: Chandra Devireddy <cdevired@codeaurora.org>