usb: ehci-omap: Show fatal probing time errors to end user

There are a few error paths in ehci_hcd_omap_probe that can be triggered
because of memory allocation or hw failure. Change those dev_dbg error
prints to dev_err with an error code printed so that the end users are able
to notice the issue.

Signed-off-by: Jarkko Nikula <jhnikula@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>