net: hsr: Create and export hsr_get_port_ndev()

Create an API to get the net_device to the slave port of HSR device. The
API will take hsr net_device and enum hsr_port_type for which we want the
net_device as arguments.

This API can be used by client drivers who support HSR and want to get
the net_devcie of slave ports from the hsr device. Export this API for
the same.

This API needs the enum hsr_port_type to be accessible by the drivers using
hsr. Move the enum hsr_port_type from net/hsr/hsr_main.h to
include/linux/if_hsr.h for the same.

Signed-off-by: MD Danish Anwar <danishanwar@ti.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>