ata: pata_cs5535: Fix W=1 warnings

x86_64 allmodconfig build with W=1 gives these warnings:

drivers/ata/pata_cs5535.c: In function ‘cs5535_set_piomode’:
drivers/ata/pata_cs5535.c:93:11: error: variable ‘dummy’ set but not
used [-Werror=unused-but-set-variable]
  u32 reg, dummy;
           ^~~~~
drivers/ata/pata_cs5535.c: In function ‘cs5535_set_dmamode’:
drivers/ata/pata_cs5535.c:132:11: error: variable ‘dummy’ set but not
used [-Werror=unused-but-set-variable]
  u32 reg, dummy;
           ^~~~~
cc1: all warnings being treated as errors

Mark variables 'dummy' as "maybe unused" as they are only ever written
in rdmsr() calls.

Signed-off-by: John Garry <john.garry@huawei.com>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>