Bluetooth: hci_sync: Use cmd->num_cis instead of magic number

Bluetooth: hci_sync: Use cmd->num_cis instead of magic number

At the moment of the check, `cmd->num_cis` holds the value of 0x1f,
which is the max number of elements in the `cmd->cis[]` array at
declaration, which is 0x1f.

So, avoid using 0x1f directly, and instead use `cmd->num_cis`. Similarly
to this other patch[1].

Link: https://lore.kernel.org/linux-hardening/ZivaHUQyDDK9fXEk@neat/
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>