serial: nulldev: Implement "pending" function to fix tstc return "true"

serial: nulldev: Implement "pending" function to fix tstc return "true"

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>