powerpc/xmon: Fiddle xmon_depth_to_print logic in xmon_show_stack()

Currently xmon_depth_to_print is static and global, but it's only
ever used in xmon_show_stack().

At least with a modern compiler it's inlined, so there's no point
in it being static, we could #define it but it's only used in one
place.

By reworking the logic we can drop count and just decrement the
max value as a loop counter. Also switch to a while loop so we
actually print no more than 64 frames as you'd expect based on the
variable name.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>