net: stmmac: Fix unused values warnings

The commit 8532f613 ("net: stmmac: introduce MSI Interrupt routines
for mac, safety, RX & TX") introduced the converity warnings:-

  1. Unused value (UNUSED_VALUE)
     assigned_value: Assigning value REQ_IRQ_ERR_MAC to irq_err here,
     but that stored value is not used.

  2. Unused value (UNUSED_VALUE)
     assigned_value: Assigning value REQ_IRQ_ERR_NO to irq_err here,
     but that stored value is overwritten before it can used.

  3. Unused value (UNUSED_VALUE)
     assigned_value: Assigning value REQ_IRQ_ERR_WOL to irq_err here,
     but that stored value is not used.

Fixed these by removing the unnecessary value assignments.

Fixes: 8532f613

 ("net: stmmac: introduce MSI Interrupt routines for mac, safety, RX & TX")
Signed-off-by: Wong Vee Khee <vee.khee.wong@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>