ata: pata_via: fix sloppy typing in via_do_set_mode()

The local variables 'T' and 'UT' are needlessly declared as *unsigned*
*long* -- the corresponding parameters of ata_timing_compute() are both
declared as *int*.  While fixing up those declarations, also make the
'via_clock' and 'T' variables *const* as they are never re-assigned
after initialization -- the object code should remain the same as gcc
previously used copy propagation anyway...

Found by Linux Verification Center (linuxtesting.org) with the SVACE static
analysis tool.

Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>