xen/balloon: simplify test for the end of usable RAM

When initializing the balloon only max_pfn needs to be checked
(max_pfn will always be <= e820_end_of_ram_pfn()) and improve the
confusing comment.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>