kernel: add missing CONFIG_NET_DSA_TAG_BRCM_LEGACY config symbol

kernel: add missing CONFIG_NET_DSA_TAG_BRCM_LEGACY config symbol

This fixes:
Distributed Switch Architecture (NET_DSA) [Y/n/m/?] y
  Tag driver for Atheros AR9331 SoC with built-in switch (NET_DSA_TAG_AR9331) [N/m/y/?] n
  Tag driver for Broadcom switches using in-frame headers (NET_DSA_TAG_BRCM) [N/m/y/?] n
  Tag driver for Broadcom legacy switches using in-frame headers (NET_DSA_TAG_BRCM_LEGACY) [N/m/y/?] (NEW)
Error in reading or end of file.

Fixes: 8fa1b576bbb5 ("linux: update b53 upstream driver")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>