Bluetooth: Controller: Fix BT_CTLR_FAST_ENC Kconfig help text

Fix BT_CTLR_FAST_ENC Kconfig help text to reflect the
current Encryption Setup Procedure behavior.

With the split architecture, Encryption Setup Procedure
will take 4 connection events when BT_CTLR_FAST_ENC is
enabled, in comparison to 3 connection events in the
legacy Controller architecture. This is due to split
architecture processes control procedures in the lower
priority Upper Linker Layer execution context.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>