bcm63xx_enet: consolidate rx SKB ring cleanup code

The rx SKB ring use the same code for cleanup at various points.
Combine them into a function to reduce lines of code.

Signed-off-by: Sieng Piaw Liew <liew.s.piaw@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>