scsi: megaraid_sas: Fix a compilation warning

The commit de516379 ("scsi: megaraid_sas: changes to function
prototypes") introduced a comilation warning due to it changed the function
prototype of read_fw_status_reg() to take an instance pointer instead, but
forgot to remove an unused variable.

drivers/scsi/megaraid/megaraid_sas_fusion.c: In function
'megasas_fusion_update_can_queue':
drivers/scsi/megaraid/megaraid_sas_fusion.c:326:39: warning: variable
'reg_set' set but not used [-Wunused-but-set-variable]
  struct megasas_register_set __iomem *reg_set;
                                       ^~~~~~~
Fixes: de516379 ("scsi: megaraid_sas: changes to function prototypes")
Signed-off-by: Qian Cai <cai@lca.pw>
Acked-by: Sumit Saxena <sumit.saxena@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>