The dma_pool_destroy() function tests whether its argument is NULL
and then returns immediately. Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Julia Lawall <julia.lawall@lip6.fr>
Cc: Mike Waychison <mikew@google.com>
Cc: Michel Lespinasse <walken@google.com>
Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>