scsi: pm8001: Mark symbols static where possible

We get 2 warnings when building kernel with W=1:
drivers/scsi/pm8001/pm8001_sas.c:530:23: warning: no previous prototype for 'pm8001_alloc_dev' [-Wmissing-prototypes]
drivers/scsi/pm8001/pm8001_hwi.c:4495:5: warning: no previous prototype for 'pm8001_chip_phy_stop_req' [-Wmissing-prototypes]

In fact, these functions are only used in the file in which they are
declared and don't need a declaration, but can be made static.  So this
patch marks these functions with 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Acked-by: Lindar Liu <lindar_liu@usish.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>