ata: pata_sil680: fix result type of sil680_sel{dev|reg}()

sil680_sel{dev|reg}() return a PCI config space address but needlessly
use the *unsigned long* type for that,  whereas the PCI config space
accessors take *int* for the address parameter.  Switch these functions
to returning *int*, updating the local variables at their call sites.
Get rid of the 'base' local variables in these functions, while at it...

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>