enetc: Fix tx rings bitmap iteration range, irq handling

The rings bitmap of an interrupt vector encodes
which of the device's rings were assigned to that
interrupt vector.
Hence the iteration range of the tx rings bitmap
(for_each_set_bit()) should be the total number of
Tx rings of that netdevice instead of the number of
rings assigned to the interrupt vector.
Since there are 2 cores, and one interrupt vector for
each core, the number of rings asigned to an interrupt
vector is half the number of available rings.
The impact of this error is that the upper half of the
tx rings could still generate interrupts during napi
polling.

Fixes: d4fd0404 ("enetc: Introduce basic PF and VF ENETC ethernet drivers")
Signed-off-by: Claudiu Manoil <claudiu.manoil@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>