[SCSI] fcoe, libfc: adds enable/disable for fcoe interface

This is to allow fcoemon util to enable or disable a fcoe interface
according to DCB link state change.

Adds sysfs module param enable and disable for this and also
updates existing other module param description to be consistent
and more accurate since older description had double "fcoe" word
with less meaningful netdev reference to user space.

Adds code to ignore redundant fc_lport_enter_reset handling for a
already disabled fcoe interface by checking LPORT_ST_DISABLED
or LPORT_ST_LOGO states, this also prevents lport state transition
on link flap on a disabled interface.

Above changes required lport state transition to get out of
disabled or logo state on call to fc_fabric_login.

Signed-off-by: Vasu Dev <vasu.dev@intel.com>
Signed-off-by: Robert Love <robert.w.love@intel.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>