[U-Boot] "tftp: adjust settings to be suitable for 100Mbit ethernet" breaks imx6

[Peng Fan <b51431@freescale.com>]

Hi,

With the following patch, I met tftp errors on mx6qsabresd and mx6ul boards:
"
=> setenv serverip 10.193.102.217; setenv ipaddr 10.193.102.31;tftp ${loadaddr} zImage
Using FEC device
TFTP from server 10.193.102.217; our IP address is 10.193.102.31
Filename 'zImage'.
Load address: 0x12000000
Loading: *
TFTP error: 'Failure to negotiate RFC1782 options' (8)
Starting again
"

Without the following patch, tftp work fine on mx6qsabresd and mx6ul board.

"
commit 620776d734e4b126c407f636bda825a594a17723
Author: Pavel Machek <pavel@denx.de>
Date:   Tue Aug 18 14:34:26 2015 +0200

    tftp: adjust settings to be suitable for 100Mbit ethernet
	    
    Adjust timouts and retry counts to be suitable for loaded ethernet
    network. With 5 seconds timeout, 10 retries maximum, tftp is
    impossible even on local network with single full-speed TCP
    connection.
						    
    100msec timeout should be suitable for most networks tftp is used on,
    that is local ethernets. Timeout count really needs to be way higher,
    as lost packets are normal when TCP is running over the same network.
										    
    Enforce 10msec minimum.
												    
    Signed-off-by: Pavel Machek <pavel@denx.de>
    Acked-by: Joe Hershberger <joe.hershberger@ni.com>
"

Regards,
Peng.
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