opp: core: remove useless mutex protection in _allocate_opp_table

Mutex protection in not needed in _allocate_opp_table as it this function
is already called under mutex lock. Furthermore, it fixes the following
issue seen with CONFIG_PROVE_LOCKING flag:

[    3.648912] =========================
[    3.648927] WARNING: held lock freed!
[    3.648944] 5.4.56-00014-g7cc4a3bdec16 #7 Not tainted
[    3.648957] -------------------------
[    3.648976] kworker/1:1/22 is freeing memory df183c00-df183fff, with a lock still held there!
[    3.648991] df183ca0 (&opp_table->lock){+.+.}, at: _opp_get_opp_table+0x1cc/0x224
[    3.649037] 5 locks held by kworker/1:1/22:
[    3.649049]  #0: df0086a0 ((wq_completion)events){+.+.}, at: process_one_work+0x200/0x858
[    3.649082]  #1: df515f1c (deferred_probe_work){+.+.}, at: process_one_work+0x200/0x858
[    3.649110]  #2: d1ff08cc (&dev->mutex){....}, at: __device_attach+0x34/0x17c
[    3.649145]  #3: c139e5e0 (opp_table_lock){+.+.}, at: _opp_get_opp_table+0x1c/0x224
[    3.649175]  #4: df183ca0 (&opp_table->lock){+.+.}, at: _opp_get_opp_table+0x1cc/0x224

Fixes: cf1f361 ("opp: core: fix memory leak in probe deferral")

Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>
Change-Id: I6da51fab26b698bb803a3a8ddf5454f73d8af956
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/linux-stm32/+/178474
Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>

Author: atorgue

drivers/opp/core.c

@@ -985,10 +985,8 @@ static struct opp_table *_allocate_opp_table(struct device *dev, int index)
 				ret);
 		}
 		else {
-			mutex_lock(&opp_table->lock);
 			_remove_opp_dev(opp_dev, opp_table);
 			kfree(opp_table);
-			mutex_unlock(&opp_table->lock);
 			return ERR_PTR(-EPROBE_DEFER);
 		}
 	}