net: mvpp2x: remove TX drain from transmit routine

TX drain in transmit procedure could cause issues due
to race between drain procedure and transmition of descriptor
between AGGR TXQ and physical TXQ.
TXQ will be cleared before moving to Linux by stop procedure.

Signed-off-by: Stefan Chulski <stefanc@marvell.com>
Tested-by: iSoC Platform CI <ykjenk@marvell.com>
Reviewed-by: Nadav Haklai <nadavh@marvell.com>
Reviewed-by: Igal Liberman <igall@marvell.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Stefan Roese <sr@denx.de>