tty: hvc: dcc: Detect JTAG connection only on CPU core 0

On SMP, migration can occur inside hvc_dcc_check() during device_initcall,
so the CPU writing the probe character and the CPU reading the TXfull
status may not be the same, which can hang the system.

Commit 4419da5d5d4b1788568b ("tty: hvc: dcc: Bind driver to CPU core0
for reads and writes") cached the DCC availability detected on core 0
during console_initcall. However, this is not sufficient: if core 0
does not detect DCC as available and another CPU later calls
hvc_dcc_check(), the check is not actually performed on core 0.

Since JTAG is not expected to be plugged or unplugged during kernel
boot, especially between console_initcall and device_initcall, checking
DCC availability multiple times is unnecessary. Cache the return value
from hvc_dcc_console_init and reuse it in hvc_dcc_init. This avoids
complex SMP-related code and ensures the result is always from core 0.

Signed-off-by: Jiannan Du <shatyuka@qq.com>
Link: https://patch.msgid.link/tencent_77968AD30F111278CBAE934BCFD0AF00E807@qq.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>