scsi: qla2xxx: Add support for ZIO6 interrupt threshold

Add sysfs support to control zio6 interrupt threshold. Using this sysfs hook
user can set when to generate interrupts. This value will be used to tell
firmware to generate interrupt at a certain interval.  If the number of
exchanges/commands fall below defined setting, then the interrupt will be
generated immediately by the firmware.

By default ZIO6 will coalesce interrupts to a specified interval
regardless of low traffic or high traffic.

[mkp: fixed several typos]

Signed-off-by: Quinn Tran <quinn.tran@cavium.com>
Signed-off-by: Himanshu Madhani <himanshu.madhani@cavium.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>