drm/i915/guc: Drop union guc_log_control

Usually we use shift/mask macros for bit field definitions.
Union guc_log_control was not following that pattern.

Additional bonus:

add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-25 (-25)
Function                                     old     new   delta
intel_guc_log_level_set                      388     363     -25

v2: prevent out-of-range verbosity (MichalWi)

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Michal Winiarski <michal.winiarski@intel.com>
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Reviewed-by: MichaĹ Winiarski <michal.winiarski@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180320181419.35576-2-michal.wajdeczko@intel.com