e1000: cleanup process_tx_desc

Coverity complains about two overruns in process_tx_desc(). The
complaints are false positives, but we might as well eliminate
them. The problem is that "hdr" is defined as an unsigned int,
but then used to offset an array of size 65536, and another of
size 256 bytes. hdr will actually never be greater than 255
though, as it's assigned only once and to the value of
tp->hdr_len, which is an uint8_t. This patch simply gets rid of
hdr, replacing it with tp->hdr_len, which makes it consistent
with all other tp member use in the function.

v2:
 - also cleanup coding style issues in the touched lines

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
