board: freescale: p1_p2_rdb_pc: Fix env $vscfw_addr

Do not stringify env $vscfw_addr two times (once implicitly via string
operator "" and second time explicitly via __stringify() macro) and allow
to compile U-Boot without CONFIG_VSC7385_ENET (when __VSCFW_ADDR was not
defined and so macro name was stringified into CONFIG_EXTRA_ENV_SETTINGS).

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>