bluetooth: controller: openisa/RV32M1: fix LL scheduling

- avoid spourious radio interrupts by fixing ISR set,
    waiting for idle, command configuration
- adjust counter to account for missing increment
- change preemption instant to avoid missing the deadline in LLL
- decrese EVENT_JITTER_US and
    EVENT_TICKER_RES_MARGIN_US (same as Nordic)

Continuous scanning and connections are working fine now.

Signed-off-by: George Stefan <george.stefan@nxp.com>