am335x: cpsw: optimize cpsw_recv to increase network performance

In 48ec5291, only TX path was optimized; this does the same also for RX
path. This results in huge increase of TFTP throughput on custom am3352
board (from 312KiB/s to 1.8MiB/s) and eliminates occasional transfer
timeouts.

Signed-off-by: Vladimir Koutny <vladimir.koutny@streamunlimited.com>
Cc: Mugunthan V N <mugunthanvnm@ti.com>
Cc: Joe Hershberger <joe.hershberger@gmail.com>
Cc: Tom Rini <trini@ti.com>