aperture_64.c: clarify that too small aperture is valid reason for this code

Impact: update comment

Clarify that too small aperture is valid reason for this code.
Signed-off-by: Pavel Machek <pavel@suse.cz>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/aperture_64.c

 /*
  * Firmware replacement code.
  *
- * Work around broken BIOSes that don't set an aperture or only set the
- * aperture in the AGP bridge.
+ * Work around broken BIOSes that don't set an aperture, only set the
+ * aperture in the AGP bridge, or set too small aperture.
+ *
  * If all fails map the aperture over some low memory.  This is cheaper than
  * doing bounce buffering. The memory is lost. This is done at early boot
  * because only the bootmem allocator can allocate 32+MB.