usb: ehci-msm-hsic: Fix compilation error

The ehci-msm-hsic driver's bus suspend and resume functions
depend on ehci_hcd's bus_suspend/resume functions, which are
NULL when CONFIG_PM is not defined. Fix the compilation error
by also #defining these sub-driver functions NULL, as they
are effectively no-ops.

Change-Id: Ie250dfae3caccb737ce26bdda17c5b21d358eced
Signed-off-by: Jack Pham <jackp@codeaurora.org>