iio: gyro: itg3200 switch sampling frequency attr to core support.

By using the info_mask_shared_by_all element of the channel spec, access
to the sampling frequency becomes available to in kernel users of the
driver.  It also shortens and simplifies the code a little.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Reviewed-by: Hartmut Knaack <knaack.h@gmx.de>
Cc: Manuel Stahl <manuel.stahl@iis.fraunhofer.de>
Cc: Thorsten Nowak <thorsten.nowak@iis.fraunhofer.de>
Cc: Christian Strobel <christian.strobel@iis.fraunhofer.de>