staging: rtl8192e: Remove unsupported mode IW_MODE_ADHOC

Remove unsupported mode IW_MODE_ADHOC because this mode is only partially
coded in the driver.

rtllib_rx_mgt() can only handle probe requests in IW_MODE_ADHOC and no
authentication or association frames.

Signed-off-by: Philipp Hortmann <philipp.g.hortmann@gmail.com>
Link: https://lore.kernel.org/r/20230827132704.GA12506@matrix-ESPRIMO-P710


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>