net: broadcom: Fix manufacturer name in Kconfig help text

The help text refers to AMD instead of Broadcom, presumably because it
was copied from the former.

Fixes: adfc5217

 ("broadcom: Move the Broadcom drivers")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: David S. Miller <davem@davemloft.net>