atl1: simplify tx packet descriptor

The transmit packet descriptor consists of four 32-bit words, with word 3
upper bits overloaded depending upon the condition of its bits 3 and 4.
The driver currently duplicates all word 2 and some word 3 register bit
definitions unnecessarily and also uses a set of nested structures in its
definition of the TPD without good cause. This patch adds a lengthy
comment describing the TPD, eliminates duplicate TPD bit definitions,
and simplifies the TPD structure itself. It also expands the TSO check
to correctly handle custom checksum versus TSO processing using the revised
TPD definitions. Finally, shorten some variable names in the transmit
processing path to reduce line lengths, rename some variables to better
describe their purpose (e.g., nseg versus m), and add a comment or two
to better describe what the code is doing.

Signed-off-by: Jay Cliburn <jacliburn@bellsouth.net>
Acked-by: Chris Snook <csnook@redhat.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>