hwmon: (w83793) VID and VRM handling cleanups

* Rework the device initialization function so as to read the
"Multi-Function Pin Control" register (0x58) once instead of twice.
I2C transactions aren't cheap so this speeds up the driver loading.

* Only create the "vrm" attribute if at least one VID value is
available.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Gong Jun <jgong@winbond.com>
Acked-by: Rudolf Marek <r.marek@assembler.cz>
Signed-off-by: Mark M. Hoffman <mhoffman@lightlink.com>