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>

Author: John Garry

drivers/ata/pata_cs5535.c

@@ -90,7 +90,7 @@ static void cs5535_set_piomode(struct ata_port *ap, struct ata_device *adev)
 	static const u16 pio_cmd_timings[5] = {
 		0xF7F4, 0x53F3, 0x13F1, 0x5131, 0x1131
 	};
-	u32 reg, dummy;
+	u32 reg, __maybe_unused dummy;
 	struct ata_device *pair = ata_dev_pair(adev);
 
 	int mode = adev->pio_mode - XFER_PIO_0;
@@ -129,7 +129,7 @@ static void cs5535_set_dmamode(struct ata_port *ap, struct ata_device *adev)
 	static const u32 mwdma_timings[3] = {
 		0x7F0FFFF3, 0x7F035352, 0x7F024241
 	};
-	u32 reg, dummy;
+	u32 reg, __maybe_unused dummy;
 	int mode = adev->dma_mode;
 
 	rdmsr(ATAC_CH0D0_DMA + 2 * adev->devno, reg, dummy);