bluetooth: controller: nrf5: simplify preprocessor conditionals

We make a minor simplification in the way we define
RADIO_PDU_LEN_MAX in Nordic nRF LE radio driver; as
nRF51 SoC series is the only series where the LL PDU
max length representation is limited to a 5-bit field,
we simplify the conditional preprocessor expression,
to avoid having to list all nRF5x SoCs with PDU LEN
MAX is represented by an 8-bit integer field.

In addition, we modify the preprocessor conditional,
that compiles in the CCM support for 8-bit length field,
PDUs to be an #ifndef CONFIG_SOC_SERIES_NRF51X, instead
of #ifdef CONFIG_SOC_COMPATIBLE_NRF52X; this will
simplify the expression when adding nRF53X series
support.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>