sunxi: mk802_defconfig: Remove SYS_EXTRA_OPTIONS

The USB_EHCI configuration is already set in this defconfig
using kconfig's config. This configuration in SYS_EXTRA_OPTIONS
must be removed and so the SYS_EXTRA_OPTIONS.

Signed-off-by: Mylène Josserand <mylene.josserand@free-electrons.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>