staging: rtl8188eu: Introduce monitor interface for IEEE 802.11 frames

This adds support for monitoring IEEE 802.11 Data and Management frames
received or transmitted by a RTL8188EU-based device handled by this
driver.

The monitor interface is not enabled by default and will be registered
only if monitor_enable module parameter is set to 1.  When enabled it
will show up as a monX network device, which can be used by the
userspace programs for monitoring network traffic.

It is intended as an exploratory/debugging tool for rtl8188eu driver.

Signed-off-by: Jakub Sitnicki <jsitnicki@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>