usb: ehci-mxc: Remove unused 'echi' variable

Since commit c73cee71 (USB: EHCI: remove ehci_port_power() routine), the
'ehci' variable is no longer used, so remove it and fix the following
build warning:

drivers/usb/host/ehci-mxc.c:41:19: warning: unused variable 'ehci' [-Wunused-variable]

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>