[libata] pata_it821x: fix warning

Reported by Andrew Morton:
drivers/ata/pata_it821x.c: In function 'it821x_port_start':
drivers/ata/pata_it821x.c:609: warning: 'mtype' may be used uninitialized in
this function

Pretty horrid fix, but so's a warning..

Signed-off-by: Jeff Garzik <jgarzik@redhat.com>