ntb: idt: Alter temperature read method

In order to create a hwmon interface for the IDT PCIe-switch temperature
sensor the already available reader method should be improved. Particularly
we need to redesign it so one would be able to read temperature/offset
values from registers of the passed types. Since IDT sensor interface
provides temperature in unsigned format 0:7:1 (7 bits for real value
and one for fraction) we also need to have helpers for the typical sysfs
temperature data type conversion to and from this format. Even though
the IDT PCIe-switch provided temperature offset got the same but signed
type it can be translated by these methods too.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>