We have macro TOKEN_MAX_RETRIES for the number of token generate retries,
so using TOKEN_MAX_RETRIES in subflow_check_req().

And rename TOKEN_MAX_RETRIES to MPTCP_TOKEN_MAX_RETRIES as it is now
exposed.

Fixes: 535fb815 ("mptcp: token: move retry to caller")
Reviewed-by: NMatthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: NJianguo Wu <wujianguo@chinatelecom.cn>
Signed-off-by: NMat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>