scsi: mpt3sas: Introduce module parameter to override 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
protocol-specific value set by the driver (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
driver default.

[mkp: tweaked commit desc]

Signed-off-by: Sreekanth Reddy <sreekanth.reddy@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>