ena: add mbuf defragmentation

[mk_semihalf.com]

Summary

When mbuf chain is too long and device cannot handle that number of
segments in DMA transaction, mbuf chain will be defragmented.
Initially, driver was dropping all mbuf chains that were exceeding
supported number of segments.

[zbb]

sys/dev/ena/ena.c

2657

Try to use boolean expressions in conditions such as rc != 0