MIPS: ath79: sort case statements in ath79_detect_sys_type

Sort the case statements alphabetically in order to improve
readability.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Luis R. Rodriguez <mcgrof@qca.qualcomm.com>
Cc: linux-mips@linux-mips.org
Cc: mcgrof@infradead.org
Patchwork: https://patchwork.linux-mips.org/patch/3505/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>