modbus: fix support for floating point values

The Modbus protocol object types are either single-bit or 16-bit word.
Other types are not defined in the specification. Types such as float
are typically mapped to two 16-bit registers. Current implementaiton
does not maps correctly to the 16-bit word addresses. On the client
side, the implementation must take into account that the number of
requested registers (Quantity of Registers) is double that of a "float"
register.  The server side should not treat "Quantity of Registers" and
"Byte count" differently for addresses reserved for floating values,
only in the user callback the two 16-bit registers are mapped to a float
value but still aligned to 16-bit register addresses.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>