e1000: fix memcpy in e1000_get_strings

drivers/net/e1000/e1000_ethtool.c:113:
#define E1000_TEST_LEN sizeof(e1000_gstrings_test) / ETH_GSTRING_LEN

drivers/net/e1000e/ethtool.c:106:
#define E1000_TEST_LEN sizeof(e1000_gstrings_test) / ETH_GSTRING_LEN

E1000_TEST_LEN*ETH_GSTRING_LEN will expand to
sizeof(e1000_gstrings_test) / (ETH_GSTRING_LEN * ETH_GSTRING_LEN)

A lack of parentheses around defines causes unexpected results due to
operator precedences.

Signed-off-by: Roel Kluin <12o3l@tiscali.nl>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>