Bluetooth: L2CAP: remove set but not used variable 'credits'

net/bluetooth/l2cap_core.c: In function l2cap_ecred_conn_req:
net/bluetooth/l2cap_core.c:5848:6: warning: variable credits set but not used [-Wunused-but-set-variable]

commit 15f02b91 ("Bluetooth: L2CAP: Add initial code for Enhanced Credit Based Mode")
involved this unused variable, remove it.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>