scsi: megaraid_sas: Introduce module parameter for default queue depth

This patch provides a module parameter and sysfs interface to select
whether the queue depth for each device should be based on the value
suggested by firmware (the default) or the maximum supported by the
controller (can_queue).

Although we have a sysfs interface per sdev to change the queue depth of
individual scsi devices, this implementation provides a single sysfs entry
per shost to switch between the controller max and the value reported by
firmware. The module parameter can provide an interface for one time grub
settings and provides persistent settings across the boot.

[mkp: tweaked commit desc]

Signed-off-by: Kashyap Desai <kashyap.desai@broadcom.com>
Signed-off-by: Chandrakanth Patil <chandrakanth.patil@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>