USB: ehci-msm2: Disable runtime pm before remove hcd

when remove callback of ehci-msm2 driver is called during unbind,
it results in crash due to unclocked access while removing hcd.
This happens due to device entered low power mode. Hence fix this
crash by disabling runtime pm before remove_hcd() call during
unbind.

Change-Id: I22840ce02a21ed677ca7c7ebf70970b768e5e05c
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>