Rework the tx credit mechanism between the cxgbe/tom driver
and the card.  This helps smooth out some burstiness in the
exchange.

Approved by:	re (glebius)
In r243868, the error message buffer errmsg have been changed from
an on-stack array to a pointer and therefore sizeof(errmsg) would
become 4 or 8 bytes depending on the architecture.

Fix this by using ERRMSGL in place of sizeof().

Submitted by:	J David <j.david.lists@gmail.com>
MFC after:	3 days
Approved by:	re (kib)