image-fit: don't set compression if it can't be read

fit_image_get_comp() should not set value -1 in case it can't read
the compression node. Instead, leave the value untouched in that case
as it can be absent and a default value previously defined by the
caller of fit_image_get_comp() should be used.

As a result the warning message
WARNING: 'compression' nodes for ramdisks are deprecated, please fix your .its file!
no longer shows if the compression node is actually absent.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Simon Glass <sjg@chromium.org>