ar71xx: base-files: always use double quotes in lib/ar71xx.sh

Be consistent and always use double quotes for case statement patterns,
as in other targets. With this approach it should be less confusing for
users adding support for new devices.

Also, be consistent with MikroTik boards wildcard pattern.

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>