staging: unisys: visorbus: add comment to explain polling logic in controlvm_periodic_work

Added a comment to explain polling frequency variation logic in
controlvm_periodic_logic() in visorchipset.c.

Signed-off-by: Sameer Wadgaonkar <sameer.wadgaonkar@unisys.com>
Reported-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: David Kershner <david.kershner@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>