hwmon: (adt7475) Potential error pointer dereferences

The adt7475_update_device() function returns error pointers.  The
problem is that in show_pwmfreq() we dereference it before the check.
And then in pwm_use_point2_pwm_at_crit_show() there isn't a check at
all.  I don't know if it's required, but it silences a static checker
warning and it's doesn't hurt anything to check.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Tokunori Ikegami <ikegami@allied-telesis.co.jp>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>