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

/*

 * 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.