iio: chemical: scd30: Cleanup initializations and fix sign-extension bug

In the Linux kernel, the following vulnerability has been resolved: iio: chemical: scd30: Cleanup initializations and fix sign-extension bug Include linux/bitfield.h for FIELD_GET(). Create new macros for bit manipulation in combination with manual bit manipulation being replaced with FIELD_GET(). The current variable declaration and initializations are barely readable and use comma separations across multiple lines. Refactor the initializations so that mantissa and exp have separate declarations and sign gets initialized later. In addition (and due to the nature of the cleanup), fix a sign-extension bug where, float32 would get bitwise anded with ~BIT(31) (which is 0xFFFFFFFF7FFFFFFF) which corrupted the exponent.