USB: EHCI: fix incorrect configuration test

This patch (as1641) fixes a minor bug in ehci-hcd left over from when
the Chipidea driver was converted to the "ehci-hcd is a library"
scheme.  The test for whether the Chipidea platform driver is active
should be IS_ENABLED(), not defined().

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>