PM / devfreq: rockchip_dmc: Fix locking when rounding rate

PM / devfreq: rockchip_dmc: Fix locking when rounding rate

There's no need to take the rcu read lock when rounding rate.

This patch fixes the following BUG:
BUG: sleeping function called from invalid context at kernel/locking/mutex.c:620
in_atomic(): 0, irqs_disabled(): 0, pid: 153, name: kworker/u16:2
5 locks held by kworker/u16:2/153:
 #0:  ("%s"("devfreq_wq")){......}, at: [<ffffff80080b8cf4>] process_one_work+0x1c4/0x58c
 #1:  ((&(&devfreq->work)->work)){......}, at: [<ffffff80080b8cf4>] process_one_work+0x1c4/0x58c
 #2:  (&devfreq->lock){......}, at: [<ffffff80089534c8>] devfreq_monitor+0x28/0x8c
 #3:  (&vop->vop_lock){......}, at: [<ffffff80084c826c>] dmc_notifier_call+0x14/0x34
 #4:  (rcu_read_lock){......}, at: [<ffffff80089557f0>] rockchip_dmcfreq_target+0x0/0x2e0
CPU: 3 PID: 153 Comm: kworker/u16:2 Not tainted 4.4.77 #2573
Hardware name: Rockchip Sheep board (DT)
Workqueue: devfreq_wq devfreq_monitor
Call trace:
[<ffffff8008089930>] dump_backtrace+0x0/0x1c8
[<ffffff8008089b0c>] show_stack+0x14/0x1c
[<ffffff800839718c>] dump_stack+0x8c/0xac
[<ffffff80080c8d5c>] ___might_sleep+0x11c/0x128
[<ffffff80080c8ddc>] __might_sleep+0x74/0x84
[<ffffff8008c371a4>] mutex_lock_nested+0x4c/0x39c
[<ffffff80089458d8>] clk_prepare_lock+0x58/0xc8
[<ffffff8008946ec8>] clk_round_rate+0x34/0x94
[<ffffff800895589c>] rockchip_dmcfreq_target+0xac/0x2e0
[<ffffff80089533f4>] update_devfreq+0x100/0x1ac
[<ffffff80089534d0>] devfreq_monitor+0x30/0x8c
[<ffffff80080b8e1c>] process_one_work+0x2ec/0x58c
[<ffffff80080ba16c>] worker_thread+0x300/0x428
[<ffffff80080bf3e0>] kthread+0x104/0x10c
[<ffffff8008082840>] ret_from_fork+0x10/0x50

Change-Id: I31f75a55da72cab597796edd5c339222094fff97
Signed-off-by: Finley Xiao <finley.xiao@rock-chips.com>