regulator: lp3972: Silence uninitialized variable warning

This is harmless but my static checker complains that "tmp" is
uninitialized if lp3972_i2c_read() fails.  I have moved the line of code
below the error handling to silence the warning.

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