sysreset: Fix unsupported request return values

The description of the sysreset request method in <sysreset.h> says that
the return value should be -EPROTONOSUPPORT if the requested reset type
is not supported by this device.

Signed-off-by: Paul Barker <paul.barker.ct@bp.renesas.com>
Reviewed-by: Simon Glass <sjg@chromium.org>