net: stmmac: removed unused variable

Add support to remove unused variables.

Change-Id: I83c3ff5b708506e5f59f955dd08b4968f2e93b51
Acked-by: Nagarjuna Chaganti <nchagant@qti.qualcomm.com>
Signed-off-by: Suraj Jaiswal <jsuraj@codeaurora.org>