realtek: refactor net rx interrupt handler rtl83xx_net_irq()

realtek: refactor net rx interrupt handler rtl83xx_net_irq()

Cleanup the code of the RTL83xx packet receive interrupt handler. Not
only for better readability but to avoid inconsistencies and stalls on
the RTL839x targets.

The current implementation seems to come from the GPL source code.
Calling the existing cleanup() function inside the interrupt context
without any locks conflicts with SMP & NAPI polling and makes things
worse instead of giving any benefit. Simply ignore RX buffer overruns
and let the device handle packet dropping itself.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/18855
Signed-off-by: Robert Marko <robimarko@gmail.com>