EHCI: msm2: Do not override ehci_irq

On new platforms core IRQ is not fired in LPM and a dedicated
irq is used to exit LPM. Hence, no need to override core's
ehci_irq handler. When this gets called it is safe to assume
that hardware is in LPM.

Change-Id: I90fdd7e9bea511f66a07386a5f9a270b102f2249
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>