net: cpsw: Add __maybe_unused to generated inlines

We generate a number of helper inline functions to make accesses easier.
However not all permutations of each function will be used and clang
will warn about unused ones.  Decorate all of them with __maybe_unused
because of this.

Cc: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>