stm class: Fix a module refcount leak in policy creation error path

Commit c7fd62bc ("stm class: Introduce framing protocol drivers")
adds a bug into the error path of policy creation, that would do a
module_put() on a wrong module, if one tried to create a policy for
an stm device which already has a policy, using a different protocol.
IOW,

| mkdir /config/stp-policy/dummy_stm.0:p_basic.test
| mkdir /config/stp-policy/dummy_stm.0:p_sys-t.test # puts "p_basic"
| mkdir /config/stp-policy/dummy_stm.0:p_sys-t.test # "p_basic" -> -1

throws:

| general protection fault: 0000 [#1] SMP PTI
| CPU: 3 PID: 2887 Comm: mkdir
| RIP: 0010:module_put.part.31+0xe/0x90
| Call Trace:
|  module_put+0x13/0x20
|  stm_put_protocol+0x11/0x20 [stm_core]
|  stp_policy_make+0xf1/0x210 [stm_core]
|  ? __kmalloc+0x183/0x220
|  ? configfs_mkdir+0x10d/0x4c0
|  configfs_mkdir+0x169/0x4c0
|  vfs_mkdir+0x108/0x1c0
|  do_mkdirat+0xe8/0x110
|  __x64_sys_mkdir+0x1b/0x20
|  do_syscall_64+0x5a/0x140
|  entry_SYSCALL_64_after_hwframe+0x44/0xa9

Correct this sad mistake by calling calling 'put' on the correct
reference, which happens to match another error path in the same
function, so we consolidate the two at the same time.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Fixes: c7fd62bc ("stm class: Introduce framing protocol drivers")
Reported-by: Ammy Yi <ammy.yi@intel.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>