Fix merging of PyUmiPacket

[azaidy]

Switchboard provides a way to merge PyUmiPacket instances here.

Currently, if the collective len of merged packets exceed 255 (umi limit), the len overflows and rolls back.
For example, if umi_len(cmd) is 254 and umi_len(other.cmd) is 5, the resulting merged packet has a len of (254+1) + (5+1) - 256 - 1 = 4 which is incorrect.

In order to support this case, the merge function must return false if the len of current packet and other packet exceed 255.