Bluetooth: controller: Fixes for BIGinfo endianness issues

The BIGinfo PDU bitsets were not accessed in a portable maner for
endianness independence, and need to be handled as bit-manipulations.

Add utility functions to access PDU bitset variables independent of
endianness. Use macros to abstract the offsets and widths.

Conversion of BIG_Handle to LE in HCI layer incorrectly use
sys_cpu_to_le16, which fail because target type is 8-bit.

Signed-off-by: Morten Priess <mtpr@oticon.com>