ata: hpt366: fix constant cast warning

gcc-5.x warns about a preexisting problem in the hpt36x pata driver:

drivers/ata/pata_hpt366.c: In function 'hpt36x_init_one':
drivers/ata/pata_hpt366.c:376:9: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-array-qualifiers]

Other ata drivers have the same problem, as ata_pci_bmdma_init_one
takes a non-const pointer, and they solve it by using a cast to
turn that pointer into a normal non-const pointer.

I also tried to change the ata core code to make host->private_data
a const pointer, but that quickly got out of hand, as some other
drivers expect it to be writable, so I ended up using the same
hack as the others here.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Tejun Heo <tj@kernel.org>