drm/i915/gmbus: fix spurious timeout on 512-byte burst reads

When reading exactly 512 bytes with burst read enabled, the
extra_byte_added path breaks out of the inner do-while without
decrementing len. The outer while(len) then re-enters and gmbus_wait()
times out since all data has been delivered. Decrement len before the
break so the outer loop terminates correctly.

Fixes: d5dc0f4 ("drm/i915/gmbus: Enable burst read")
Signed-off-by: Samasth Norway Ananda <samasth.norway.ananda@oracle.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patch.msgid.link/20260316231920.135438-2-samasth.norway.ananda@oracle.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
(cherry picked from commit 4ab0f09)
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

Author: samasth-norway

drivers/gpu/drm/i915/display/intel_gmbus.c

@@ -496,8 +496,10 @@ gmbus_xfer_read_chunk(struct intel_display *display,
 
 		val = intel_de_read_fw(display, GMBUS3(display));
 		do {
-			if (extra_byte_added && len == 1)
+			if (extra_byte_added && len == 1) {
+				len--;
 				break;
+			}
 
 			*buf++ = val & 0xff;
 			val >>= 8;