media: dvb-usb: m920x: Fix a potential memory leak in m920x_i2c_xfer()

'read' is freed when it is known to be NULL, but not when a read error
occurs.

Revert the logic to avoid a small leak, should a m920x_read() call fail.

Fixes: a2ab06d7

 ("media: m920x: don't use stack on USB reads")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>