[SCSI] megaraid_sas: intercept cmd timeout and throttle io

eh_timed_out call back (megasas_reset_timer) is used to throttle io
to the adapter when it is called the first time for a scmd.
The MEGASAS_FW_BUSY flag is set and can_queue reduced to 16.
The can_queue is restored from completion routine in following
two conditions : 5 seconds has elapsed and
the # of outstanding cmds in FW is < 17.

Signed-off-by: Sumant Patro <sumant.patro@lsi.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>