hwmon: (it87) Don't use pdev as static driver variable

Using the same varible name for function names and as static
variable invites misuse and prevents us from adding support
for a second chip. Rename pdev to it87_pdev and limit its use
to where it is needed.

Tested-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>