regulator: max8997: clean up a condition in max8997_list_voltage()

The current code generates a static cehcker warnings because "rid < 0"
is always false:

    drivers/regulator/max8997-regulator.c:169 max8997_list_voltage()
    warn: condition is always false

The problem is that because of type promotion, if "rid" is negative the
comparison against ARRAY_SIZE() is type promoted to size_t and it's
treated as a very high positive value.  I've changed the order of the
checks so now everyone is happy.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mark Brown <broonie@kernel.org>