Bluetooth: Host: increase BT_LONG_WQ stack size when using ECC emulation

After the switch from TinyCrypt to PSA Crypto API as crypto
backend, runtime crashes might happen on some platform due
to BT_LONG_WQ's stack size not being large enough. This
commit fixes this issue.

Signed-off-by: Valerio Setti <vsetti@baylibre.com>