The kernel-provided SCO write MTU is currently never the correct packet
size for writing, so don't try to use it. Some adapter firmware (eg.
BCM20702A0 0b05:17cb) appears in practice sensitive to the alignment of
the msbc frames, and writes with wrong packet size break things but only
on certain headsets. For other adapters, this doesn't appear to matter.
This is a bitfield, but it's unclear what it achieves since this is the
only member of a bitfield, so it may be more efficient to just make it a
bool.
Fixes a LGTM warning:
Bit field started of type int should have explicitly unsigned integral, explicitly signed integral, or enumeration type.
Move SCO polling to a single place, and abstract mtu handling.
Autodetect suitable tx packet size based on rx, instead of relying on
the kernel providing correct values.
