new return type for get_persistent_mac and set_persistent_mac

Some declaration was int8_t, some int32_t and some didn't match
the function definition. Now everything returns just int.
Anyway the functions return only 0 or -1, so there are no problems
returning int. Moreover in this way the resulting assembler is
smaller for every target architecture (even if here we always
build for lm32)

This reduces the binary by 30 bytes.