drivers: aio: Kconfig: Remove redundant 'default n' properties

Bool symbols implicitly default to 'n'.

A 'default n' can make sense e.g. in a Kconfig.defconfig file, if you
want to override a 'default y' on the base definition of the symbol. It
isn't used like that on any of these symbols though, and is
inconsistent.

This will make the auto-generated Kconfig documentation have "No
defaults. Implicitly defaults to n." as well, which is clearer than
'default n if ...'

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>