V4L/DVB (9926): em28xx: Fix a bug that were putting xc2028/3028 tuner to sleep

The changeset 78aa52a159cf introduced a bug on em28xx: buffer setup should be
awaking xc3028. Instead, since we didn't specify the tuner mode, the device
were going to sleep, due to the lack of tuner mode when asking tuner to handle
VIDIOC_S_FREQUENCY:

xc2028 0-0061: Device is Xceive 3028 version 1.0, firmware version 2.7
xc2028 0-0061: divisor= 00 00 14 d0 (freq=83.250)
xc2028 0-0061: Putting xc2028/3028 into poweroff mode.
xc2028 0-0061: Printing sleep stack trace:
Pid: 10936, comm: mplayer Tainted: P   M      2.6.27.8 #1

Call Trace:
 [<ffffffffa0b759ea>] xc2028_sleep+0x89/0x1ab [tuner_xc2028]
 [<ffffffffa0b48fb9>] tuner_s_frequency+0xf5/0x165 [tuner]

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>