audio: Ignore ENOSYS from optional legacy HAL methods

Some methods of legacy HALs are optional. The legacy HAL
can avoid implementing them either by setting the function
entry in the interface table to NULL or by setting it
to a stub function returning -ENOSYS. In the latter case
the default implementation was spamming syslog with
warnings. Specify that ENOSYS returned from optional legacy
HAL methods must be ignored.

Bug: 141989952
Test: check logcat
Change-Id: Idbdff8d88e3303a0181cb5629ab82485148e6d8e