[media] em28xx: remove some unnecessary fields from struct em28xx_audio_mode

Fields "ac97_feat", "ac97_vendor_id" and "i2s_samplerates" of struct
em28xx_audio_mode are used nowhere, except in function
em28xx_audio_setup(). So get rid of them and use local variables
instead.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>