MIPS: BCM63xx: Remove !RUNTIME_DETECT code for bcmcpu_get_id

Use the same pattern as with get_*_cpu_type() to allow the compiler
to remove code for non enabled SoC types.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: John Crispin <blogic@openwrt.org>
Cc: Maxime Bizon <mbizon@freebox.fr>
Cc: Florian Fainelli <florian@openwrt.org>
Cc: Kevin Cernekee <cernekee@gmail.com>
Patchwork: https://patchwork.linux-mips.org/patch/7273/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>