media: atomisp: Remove no longer used atomisp_css_flush()

Remove the no longer used atomisp_css_flush() function and merge
atomisp_assert_recovery_work() and __atomisp_css_recover() into
a single function.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>