drm/i915/uc: Don't use -EIO to report missing firmware

drm/i915/uc: Don't use -EIO to report missing firmware

-EIO has special meaning and is used when we want to allow
engine initialization to fail and mark GPU as wedged.

However here at this function we should return error code
that corresponds to upload status only, as any decision how
to handle missing firmware should be done higher level function
(silent fallback to non-GuC mode, fail into wedged mode, or
abort driver load with fatal error).

v2: commit message update (Michal)

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171206135316.32556-5-michal.wajdeczko@intel.com