macintosh/ams: Fix unused variable warning

If both CONFIG_SENSORS_AMS_PMU and CONFIG_SENSORS_AMS_I2C are unset,
there is an unused variable warning in the ams driver:

  drivers/macintosh/ams/ams-core.c: In function 'ams_init':
  drivers/macintosh/ams/ams-core.c:181:29: warning: unused variable 'np'
    181 |         struct device_node *np;

The driver needs at least one of the configs enabled in order to
actually function. So fix the compiler warning by ensuring at least one
of the configs is enabled.

Suggested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240507140150.54630-1-mpe@ellerman.id.au