drm/i915/uc: Rename intel_uc_fw.fw to .type

This field is used to determine which kind of firmware the struct
describes (GuC/HuC) - the name does not reflect.

The enum used here have "type" in the name, so let's go with that.

Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Oscar Mateo <oscar.mateo@intel.com>
Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170315133415.15343-1-arkadiusz.hiler@intel.com