serial-uclass: reset gd->cur_serial_dev to NULL if serial not found

serial-uclass: reset gd->cur_serial_dev to NULL if serial not found

Reset gd->cur_serial_dev pointer to avoid calling non-relocated code
from relocated code if a serial driver is not found and
CONFIG_REQUIRE_SERIAL_CONSOLE is disabled.

Here is detailed explanation of what this patch is trying to fix.

U-boot calls the serial_find_console_or_panic() function twice.
The first console setup occurs before U-boot relocation in
the serial_init(). This stage uses simple FDT parsing and
assigns gd->cur_serial_dev to a "serial" device that lives in
non-relocated code too.

The second console setup after U-boot relocation(from serial_initialize())
may use full live DT (if OF_LIVE enabled) probe sequence with buses,
clocks, resets, etc... And if the console setup fails at this step,
than we should be caught by panic_str("No serial driver found").

But... If we disable CONFIG_REQUIRE_SERIAL_CONSOLE, than we
return from serial_init() with gd->cur_serial_dev pointing
to the "old"(non-relocated) serial device.

And if this area, where "old" serial device is placed, is changed
(e.g. Linux kernel may be relocated at this address), than we will get
an unexpected crash on the next call of printf().

Signed-off-by: Maksim Kiselev <bigunclemax@gmail.com>