drivers: sensor: bmm350: nit picky cleanup

There were a lot of places where the return value was sum'ed up
which can be slightly easier to read, but can give an odd return
value in the end. This changes it to just return immediately if
an error is seen.

There was also runtime NULL checking of input arguments that are
rather programmer clown prevention. Change these to asserts.

The return value didn't have a consistant type through the file,
make it consistently an int.

This changes the name of the return value of fix_sign to be
signed_value differenating it from the ret name used elsewhere.

Use the sizeof(buffer) as the argument for reg reads and writes
rather than a hard number.

Some log messages ended with '\n'. Remove all these.

Signed-off-by: Ryan McClelland <ryanmcclelland@meta.com>