sata_sx4: Check return code from pdc20621_i2c_read()

The variable spd0 might be used uninitialized when pdc20621_i2c_read()
fails.  This also generates a compilation warning with gcc 5.1.

tj: use pr_err()

Signed-off-by: Tomer Barletz <barletz@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>