pcnet32: Remove redundant variable prev_link and curr_link

Variable prev_link and curr_link is being assigned a value from a
calculation however the variable is never read, so this redundant
variable can be removed.

Cleans up the following clang-analyzer warning:

drivers/net/ethernet/amd/pcnet32.c:2857:4: warning: Value stored to
'prev_link' is never read [clang-analyzer-deadcode.DeadStores].

drivers/net/ethernet/amd/pcnet32.c:2856:4: warning: Value stored to
'curr_link' is never read [clang-analyzer-deadcode.DeadStores].

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>