media: atomisp: Remove never set file_input flag

After the file-injection support removal the file_input flag is
always false.

Remove the flag and replace any code checking it with the code-path
for when it is false.

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