hwmon: (w83791d) rename prototype parameter from 'register' to 'reg'

gcc -Wextra warns "register is not at beginning of declaration" because the
compiler thinks the parameter has been marked as a 'register' variable, but
the function prototype intended to name the parameter "register" (which is a
reserved keyword).

Signed-off-by: Chris Peterson <cpeterso@cpeterso.com>
Acked-by: Marc Hulsman <m.hulsman@tudelft.nl>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>