[SCSI] fc class: add fc host dev loss sysfs file

This adds a fc host dev loss sysfs file. Instead of
calling into the driver using the get_host_def_dev_loss_tmo
callback, we allow drivers to init the dev loss like is done
for other fc host params, and then the fc class will handle
updating the value if the user writes to the new sysfs file.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>