stmmac: remove spin_lock in stmmac_ioctl.

stmmac: remove spin_lock in stmmac_ioctl.

This patch removes un-needed spin_lock in stmmac_ioctl while reading and
writing mdio registers. While holding spin_lock the code must be
atomic, which is not true in this case as both mdiobus_read and writes
have mutex locks.

Without this patch reading mdio registers via mii-tool results in below
BUG:
mii-tool -vvv eth0"
Using SIOCGMIIPHY=0x8947
BUG: sleeping function called from invalid context at kernel/mutex.c:287
in_atomic(): 1, irqs_disabled(): 0, pid: 614, name: mii-tool
2 locks held by mii-tool/614:
 #0:  (rtnl_mutex){......}, at: [<c01fd80c>] dev_ioctl+0x550/0x674
 #1:  (&priv->lock){......}, at: [<c01b34ec>] stmmac_ioctl+0x4c/0x78
[<c002ea14>] (unwind_backtrace+0x0/0xcc) from [<c0272c38>]
(mutex_lock_nested+0x24/0x35c)
[<c0272c38>] (mutex_lock_nested+0x24/0x35c) from [<c01b237c>]
(mdiobus_read+0x44/0x70)
[<c01b237c>] (mdiobus_read+0x44/0x70) from [<c01b0c64>]
(phy_mii_ioctl+0x4c/0x138)
[<c01b0c64>] (phy_mii_ioctl+0x4c/0x138) from [<c01b34fc>]
(stmmac_ioctl+0x5c/0x78)
[<c01b34fc>] (stmmac_ioctl+0x5c/0x78) from [<c01fcec8>]
(dev_ifsioc+0x2a4/0x2c8)
[<c01fcec8>] (dev_ifsioc+0x2a4/0x2c8) from [<c01fd81c>]
(dev_ioctl+0x560/0x674)
[<c01fd81c>] (dev_ioctl+0x560/0x674) from [<c00c36e0>]
(vfs_ioctl+0x2c/0x8c)
[<c00c36e0>] (vfs_ioctl+0x2c/0x8c) from [<c00c4130>]
(do_vfs_ioctl+0x530/0x578)
[<c00c4130>] (do_vfs_ioctl+0x530/0x578) from [<c00c41ac>]
(sys_ioctl+0x34/0x54)
[<c00c41ac>] (sys_ioctl+0x34/0x54) from [<c0028aa0>]
(ret_fast_syscall+0x0/0x2c)

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>