bcm63xx_enet: improve rx loop

Use existing rx processed count to track against budget, thereby making
budget decrement operation redundant.

rx_desc_count can be calculated outside the rx loop, making the loop a
bit smaller.

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>