drivers/net/e2100.c: fix sparse warning: symbol shadows an earlier one

Impact: Remove redundant inner scope variable and while being at it
make use of ARRAY_SIZE instead of a hardcoded number.

Fix this sparse warning:
  drivers/net/e2100.c:219:56: warning: symbol 'i' shadows an earlier one
  drivers/net/e2100.c:181:13: originally declared here

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: David S. Miller <davem@davemloft.net>