If "buf" wasn't equal to LP5521_REG_R_CURR_DEFAULT the probe fails but
we still return zero.  I've changed it to print an error message and
return -EINVAL.
Reported-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>