scsi: arcmsr: Spin off duplicate code

Spin off duplicate code of timer init for message isr BH in arcmsr_probe
and arcmsr_resume as a function arcmsr_init_get_devmap_timer.

Signed-off-by: Ching Huang <ching2048@areca.com.tw>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>