USB: EHCI: make ehci_read_frame_index platform independent

In preparation for splitting the ehci-hcd driver into a core library
and separate platform-specific driver modules, this patch (as1617)
changes the way ehci_read_frame_index() is handled.

Since the same core library will have to work with both PCI and
non-PCI platforms, the quirk handler routine will be compiled
unconditionally.  The decision about whether to call it or simply to
read the frame index register is made at run time, based on whether
the frame_index_bug quirk flag is set.

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