net: stmmac: Simplify mtl IRQ status checking

Commit 8a7cb245 ("net: stmmac: Do not enable RX FIFO overflow
interrupts") disabled the RX FIFO overflow interrupts. However, it left the
status variable around, but never checks it.

As stmmac_host_mtl_irq_status() returns only 0 now, the code can be
simplified.

Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Link: https://lore.kernel.org/r/20240208-stmmac_irq-v1-1-8bab236026d4@linutronix.de


Signed-off-by: Paolo Abeni <pabeni@redhat.com>