USB: EHCI: make ehci-pci a separate driver

This patch (as1625) splits the PCI portion of ehci-hcd out into its
own separate driver module, called ehci-pci.  Consistently with the
current practice, the decision whether to build this module is not
user-configurable.  If EHCI and PCI are enabled then the module will
be built, always.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
CC: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>