serial-uclass: set GD_FLG_SERIAL_READY only when cur_serial_dev is assigned

serial-uclass: set GD_FLG_SERIAL_READY only when cur_serial_dev is assigned

serial_find_console_or_panic() may left cur_serial_dev unassigned if
REQUIRE_SERIAL_CONSOLE is not set. Setting GD_FLG_SERIAL_READY in
this situation confuses serial console code. It tries to use
unassigned driver instead of debug port and stops printing.
So check cur_serial_dev before setting GD_FLG_SERIAL_READY to allow
console to keep printing via debug port.

Signed-off-by: Maxim Kochetkov <fido_max@inbox.ru>