kgdbts: prevent re-entry to kgdbts before it unregisters

kgdbts: prevent re-entry to kgdbts before it unregisters

The "kgdb_connected" variable of debug_core just indicates whether or
not kgdbts is connected to the debug_core.  It does not completely
prevent a script from trying invoke kgdbts again and possibly crashing
the system (see Call Trace below).

The configured variable in kgtbts can be used instead of
kgdb_connected instead of kgdb_connected. The cleanup_kgdbts() can
also be removed because there is no possible way to build kgdbts as a
kernel module that you could unload with rmmod.

Call Trace:
-----------------------------------------------------------------
root:/$ echo kgdbts=V1S1000 > /sys/module/kgdbts/parameters/kgdbts
kgdb: Unregistered I/O driver kgdbts, debugger disabled.
------------[ cut here ]------------
WARNING: at kernel/debug/debug_core.c:1002
kgdb_unregister_io_module+0xec/0x100()
Hardware name: Moon Creek platform
Modules linked in:
Pid: 664, comm: sh Not tainted 2.6.34.1-WR4.0.0.0_standard #58
Call Trace:
 [<c103b1ed>] warn_slowpath_common+0x6d/0xa0
 [<c1079fdc>] ? kgdb_unregister_io_module+0xec/0x100
 [<c1079fdc>] ? kgdb_unregister_io_module+0xec/0x100
 [<c10544e0>] ? param_attr_store+0x0/0x20
 [<c103b235>] warn_slowpath_null+0x15/0x20
 [<c1079fdc>] kgdb_unregister_io_module+0xec/0x100
 [<c124e4ea>] cleanup_kgdbts+0x1a/0x20
 [<c124eced>] param_set_kgdbts_var+0x6d/0xb0
 [<c124ec80>] ? param_set_kgdbts_var+0x0/0xb0
 [<c10544f7>] param_attr_store+0x17/0x20
 [<c105457c>] module_attr_store+0x2c/0x40
 [<c111fe84>] sysfs_write_file+0x94/0xf0
 [<c10d42f6>] vfs_write+0x96/0x130
 [<c111fdf0>] ? sysfs_write_file+0x0/0xf0
 [<c10d44d6>] sys_write+0x46/0xd0
 [<c13bf329>] system_call_done+0x0/0x4
---[ end trace 4eb028c6ee43154c ]---
kgdb: Unregistered I/O driver kgdbts, debugger disabled.
-----------------------------------------------------------------

[jason.wessel@windriver.com: remove cleanup_kgdbts() ]
Signed-off-by: Dongdong Deng <dongdong.deng@windriver.com>
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>