platform/x86: think-lmi: Fix current password length check

current_password_store() checks the password length before removing the
trailing newline, which can reject valid passwords that are exactly
->maxlen bytes long.

It also passes ->maxlen to strscpy(), which truncates passwords without
a newline.

Use strchrnul() to measure the password length up to the newline, then
copy that many bytes and add a trailing NUL terminator using strscpy().

Fixes: a40cd7ef22fb ("platform/x86: think-lmi: Add WMI interface support on Lenovo platforms")
Cc: stable@vger.kernel.org
Reviewed-by: Mark Pearson <mpearson-lenovo@squebb.ca>
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Link: https://patch.msgid.link/20260818151635.37094-2-thorsten.blum@linux.dev
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>