usb: ehci-msm2: Don't fail probe if clk_get fails for optional clocks

Clock driver API: clk_get may return EPROBE_DEFER even for the clocks
that are not defined for the platform. This makes it difficult to
find out if clk_get failure with EPROBE_DEFER was genuine or it
failed because the clock is not defined for the platform.
Take a simple approach to handle this by moving optional clocks' get
to the last and not to fail probe if clk_get fails for them.

Change-Id: Ia074c2fbd3bc58ca60e70de98c53712b72c33887
Signed-off-by: Tarun Gupta <tarung@codeaurora.org>
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>