usb: ehci-vf: use ehci_deregister() for .remove callback

This driver was recently converted to Driver Model, so missed the
subsystem-wide cleanups by commit 40527342 ("usb: replace
ehci_*_remove() with usb_deregister()").

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>