mailbox converted to pci_alloc_consistent()

- request_region() is not needed: zatm_init_one() issues
  pci_request_regions();
- the warning related to kfree(zatm_dev->mbx_start) disappears;

Compiled with i386 and sparc64 as target.
Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: David S. Miller <davem@davemloft.net>