net: stmmac: Remove unneeded I/O read caught by cppcheck

Cppcheck found a case where a local variable was being assigned a value,
but not used. There seems to be no reason to read this register before
assigning a new value, so addressing thie issue.

cppcheck --force --enable=all --inline-suppr . shows ...

Variable 'value' is reassigned a value before the old one has been used.

Signed-off-by: Vince Bridgers <vbridgers2013@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>