regulator: lp3971: Silence uninitialized variable warning

This is harmless but if lp3971_i2c_read() fails then "tmp" can be
uninitialized.  Silence the warning by moving the error handling up a
line.

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