ofono/hsphfpd: force write_mtu=24 for mSBC codec

Kernel should provide the correct mtu/packet size to use for SCO socket
I/O, but it does not currently appear to (as of linux 5.9), see
https://lore.kernel.org/linux-bluetooth/20201210003528.3pmaxvubiwegxmhl@pali/T/

When using mSBC, instead of using the (incorrect) kernel-provided value,
hardcode mtu to 24, corresponding to ALT1 mode.  BT-UART adapters should
not require specific mtu, and it appears most BT-USB adapters only
support ALT1, see
https://lore.kernel.org/linux-bluetooth/20201210012003.133000-1-tpiepho@gmail.com/

This code needs to be revised when the issue is sorted out on the kernel
side.

Add setting bluez5.msbc-force-mtu for easier debugging in special cases.

Leave read_mtu as the detected value --- it does not seem to affect
whether BT-USB can record, but Raspberry Pi BT-UART fails to record if a
different value is used.