realtek: eth: merge & simplify irq handlers

realtek: eth: merge & simplify irq handlers

Two different irq handlers exist for RTL83xx and RTL93xx. Basically
they do always the same.

- Check transmit interrupts (not needed anymore)
- Check rx overflow interrupts (not needed anymore)
- Determine rx interrupts and queues that must be processed.
- In case of RTL839x check for L2 interrupts

With all the recent refactoring their logic is more or less the
same. Merge them into one handler. For better readability add a
helper that determines the work (aka rings) that needs to be
processed.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/22023
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>