staging: r8188eu: remove HW_VAR_DM_FLAG member of hw_variables enumeration

rtw_hal_set_hwreg and rtw_hal_get_hwreg does not used
with HW_VAR_DM_FLAG parameter.

Signed-off-by: Ivan Safonov <insafonov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>