libata: Remove FIXME comment in atapi_request_sense()

Remove the FIXME comment in atapi_request_sense() asking whether
memset of sense buffer is necessary.  The buffer may be partially or
fully filled by the device.  We want it to be cleared.

tj: Updated description.

Signed-off-by: Nicholas Krause <xerofoify@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>