platform: mellanox: Fix order in exit flow

Fix exit flow order: call mlxplat_post_exit() after
mlxplat_i2c_main_exit() in order to unregister main i2c driver before
to "mlxplat" driver.

Fixes: 0170f616

 ("platform: mellanox: Split initialization procedure")
Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Reviewed-by: Michael Shych <michaelsh@nvidia.com>
Link: https://lore.kernel.org/r/20230813083735.39090-2-vadimp@nvidia.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>