ata: sata_mv: fix mis-conversion in mv_write_cached_reg()

Fix the signed issue in mv_write_cached_reg() where the laddr
is assigned from a (long)addr instead of (unsigned long)addr.

Fixes the following warnings:

drivers/ata/sata_mv.c:989:26: warning: cast removes address space of expression
drivers/ata/sata_mv.c:989:26: warning: cast removes address space of expression
drivers/ata/sata_mv.c:989:26: warning: cast removes address space of expression
drivers/ata/sata_mv.c:989:26: warning: cast removes address space of expression

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Tejun Heo <tj@kernel.org>