mx6memcal: Remove SPL_USB_HOST

As this particular platform is intended to be loaded and run a specific
set of routines in SPL, we do not need the ability to further use the
USB as a host device in SPL.  Disable this support.

Cc: Eric Nelson <eric@nelint.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Eric Nelson <eric@nelint.com>