drivers: ieee802154: cc13/26xx_subg: remove redundant configuration

The low-level configuration of the chip's radio commands was mostly
redundant. This change removes redundant configuration code.

This is also relevant as a preparation to supporting further frequency
bands and operating modes on the same SUN FSK channel page with
similar but slightly different settings (center frequencies, channel
spacing, modulation index as defined in the standard). The SUN FSK
standard defines plenty of such variations with different physical
characteristics and trade-offs. Such variations are highly relevant in
industrial applications which will be targeted by TSCH. Using the
correct settings is required for additional features (e.g. frequency
hopping) and interoperability.

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>