net: netcp: add api to support set rx mode in netcp modules

This patch adds an API to support setting rx mode in
netcp modules.  If a netcp module needs to be notified
when upper layer transitions from one rx mode to
another and react accordingly, such a module will implement
the new API set_rx_mode added in this patch.  Currently
rx modes supported are PROMISCUOUS and NON_PROMISCUOUS
modes.

Signed-off-by: WingMan Kwok <w-kwok2@ti.com>
Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>