stmmac: fix pointer check after utilization in stmmac_interrupt

The paranoidal pointer check in IRQ handler looks very strange - it
really protects us only against bogus drivers which request IRQ line
with null pointer dev_id. However, the code fragment is incorrect
because the dev pointer is used before the actual check which leads
to undefined behavior. Remove the check to avoid confusing people
with incorrect code.

Signed-off-by: Maxim Petrov <mmrmaximuzz@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>