[U-Boot] serial: nulldev: Implement "pending" function to fix tstc return "true"

[Tom Rini <trini@konsulko.com>]

On Thu, Nov 02, 2017 at 11:39:51PM -0700, Wilson Lee wrote:

> In U-boot, serial_tstc was use to determine is there have a character in
> serial console that pending for read. If there is no "pending" function
> implemented in serial driver, the serial-uclass will return "true(1)"
> to indicate there have a character pending to read.
> 
> Thus, read a character from nulldev serial will result in continuous
> getting -EAGAIN return which might lead system to hang.
> 
> This commit is to fix a bug in nulldev serial which implement "pending"
> function in nulldev serial to always indicate there is no character in
> console that pending for read.
> 
> Signed-off-by: Wilson Lee <wilson.lee@ni.com>
> Cc: Joe Hershberger <joe.hershberger@ni.com>
> Cc: Keng Soon Cheah <keng.soon.cheah@ni.com>
> Cc: Chen Yee Chew <chen.yee.chew@ni.com>
> Cc: Bin Meng <bmeng.cn@gmail.com>

Applied to u-boot/master, thanks!

-- 
Tom
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