Fixes gcc '-Wunused-but-set-variable' warning:

drivers/usb/host/ehci-st.c: In function st_ehci_platform_probe:
drivers/usb/host/ehci-st.c:155:19: warning: variable ehci set but not used [-Wunused-but-set-variable]

It is never used, so can be removed.
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>