xen: credit2: clear bit instead of skip step in runq_tickle()

xen: credit2: clear bit instead of skip step in runq_tickle()

Since we are doing cpumask manipulation already, clear a bit
in the mask at once. Doing that will save us an if, later in
the code.

No functional change intended.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>