rtlwifi: rtl8192c: Prevent sleeping from invalid context in rtl8192cu

When driver rtl8192cu is used with the debug level set to 3 or greater,
the result is "sleeping function called from invalid context" due to
an rcu_read_lock() call in the DM refresh routine in driver rtl8192c.
This lock is not necessary as the USB driver does not use the struct
being protected, thus the lock is set only when a PCI interface is
active.

This bug is reported in https://bugzilla.kernel.org/show_bug.cgi?id=42775

.

Reported-by: Ronald Wahl <ronald.wahl@raritan.com>
Tested-by: Ronald Wahl <ronald.wahl@raritan.com>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Stable <stable@vger.kernel.org>
Cc: Ronald Wahl <ronald.wahl@raritan.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>