[SCSI] expose eh_timed_out to the host template

It looks like megaraid_sas at least needs this to throttle its commands
as they begin to time out.  The code keeps the existing transport
template use of eh_timed_out (and allows the transport to override the
host if they both have this callback).

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>