Bluetooth: Controller: Fix slot_plus_us by converting max CTE len to us

Fixes #53786
The variable slot_plus_us is assumed to be a number in us unit.
To assign the maximum possible CTE length to this variable,
BT_HCI_LE_CTE_LEN_MAX has been used while this parameter is defined
in a 8us units (it's 0x14=20 that corresponds to 160us).
To convert this number to us, it's needed to be multiplied by 8.

Signed-off-by: Saleh Mehdikhani <saleh.mehdikhani@unikie.com>