UWB: Return UWB_RSV_ALLOC_NOT_FOUND rather than crashing on NULL dereference if kzalloc fails

Crashing on a null pointer deref is never a nice thing to do. It seems
to me that it's better to simply return UWB_RSV_ALLOC_NOT_FOUND if
kzalloc() fails in uwb_rsv_find_best_allocation().

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Acked-by: David Vrabel <david.vrabel@csr.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>