gpio: sandbox: Make sandbox_gpio_set_flags() set all flags

Allow this function to see all flags, including the internal sandbox ones.
This allows the tests to fully control the behaviour of the driver.

To make this work, move the setting of GPIOD_EXT_HIGH -to where the flags
are updated via driver model, rather than the sandbox 'back door'.

Signed-off-by: Simon Glass <sjg@chromium.org>

Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>