Bluetooth: mgmt: remove redundant assignment to variable cur_len

Variable cur_len is being ininitialized with a value in the start of
a for-loop but this is never read, it is being re-assigned a new value
on the first statement in the for-loop.  The initialization is redundant
and can be removed.

Cleans up clang scan build warning:
net/bluetooth/mgmt.c:7958:14: warning: Although the value stored to 'cur_len'
is used in the enclosing expression, the value is never actually read
from 'cur_len' [deadcode.DeadStores]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>