[libata] ahci: Fix warning: comparison between 'enum <anonymous>' and 'enum <anonymous>'

The below patch casts ATA_MAX_QUEUE to int because GCC will
give a warning message about the two different enum blocks:
  CC      drivers/ata/ahci.o
drivers/ata/ahci.c: In function 'ahci_init_one':
drivers/ata/ahci.c:1045:2: warning: comparison between 'enum <anonymous>' and 'enum <anonymous>'

Signed-off-by: Justin P. Mattock <justinmattock@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>