net: natsemi: Remove set but not used variable

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/ethernet/natsemi/ns83820.c: In function ns83820_get_link_ksettings:
drivers/net/ethernet/natsemi/ns83820.c:1210:11: warning: variable ‘tanar’ set but not used [-Wunused-but-set-variable]

`tanar` is never used, so remove it.

Signed-off-by: Zheng Yongjun <zhengyongjun3@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>