net: ethernet: cpsw: unroll IRQ request loop

This patch is in preparation for a nicer IRQ
handling scheme where we use different IRQ
handlers for each IRQ line (as it should be).

Later, we will also drop IRQs offset 0 and 3
because they are always disabled in this driver.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>