If only the first 't4_read_write_register()' call fails, the error code
will be overwritten and lost.
Directly report the error instead.

While at it, log some errors if 't4_read_write_register()' fails, as done
in the rest of the driver.
Signed-off-by: NChristophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>