[toble] fix softdevice config to mitigate MTU exchange error

[Hyungsin]

@turon, Thread allows MTU size up to 247 bytes, resulting in error during the MTU exchange procedure (https://b.corp.google.com/issues/142724131). Fixing two parameters resolve the problem.

[turon]

LGTM 👍

Just for reference, MTU=247 lets the full MTU fit in a single packet when BLE 4.1 DLE (Data Length Extensions) are enabled.

[jaylogue]

Help me understand the motivation for this change. issues/142724131 seems to suggest that this works around a bug in Nordic's SoftDevice. However I'm unclear why ToBLE can't handle a connection MTU set larger than 247. I would expect ToBLE to work the same way WoBLE does and be completely agnostic to the MTU.

Also, this seems like a fragile change. If 247 is somehow a magic number, its unclear how application developers would know this.

[Hyungsin]

@jaylogue, this simply comes from a value mismatch. 247 is the value used in ToBLE code.

ToBLE wants to set MTU size to 247 bytes, which is OT_BLE_ATT_MTU_MAX (https://github.com/openthread/ot-nest-nordic/blob/master-ble-cert/include/openthread/platform/ble.h#L192). But this lock app configures the softdevice to set MTU size to 251, which causes error at the first attempt of MTU exchange.

[turon]

I agree with Jay that ToBLE should be agnostic to MTU size. Can we resolve the conflict by making sure ToBLE and WoBLE are configuring the MTU to the same value through a single constant that is passed down rather than two constants that are manually set to the same value?

We may also want to extend ToBLE setup to skip mtu negotiation if MTU >= to the minimum ToBLE requires for good performance.