staging: rtl8712: remove unused array iw_operation_mode

Array iw_operation_mode is defined but is never used hence it is
redundant and can be removed. This array has been like this since
the driver was added back in 2010

Cleans up clang warning:
warning: 'iw_operation_mode' defined but not used [-Wunused-const-variable=]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>