[media] cxd2820r: tweak search algorithm behavior

MPIS based STBs running 3.x kernels and the Enigma2 OS are not able to tune
DVB-T channels with the PCTV 290e using the current cxd2820r driver.
DVB-T2 channels instead work properly.

This patch fixes the problem by changing the condition to break out from the
wait lock loop in the "search" function of the cxd2820r demodulator from
FE_HAS_SIGNAL to FE_HAS_LOCK.

As a consequence, the "search" function of the demodulator driver now returns
DVBFE_ALGO_SEARCH_SUCCESS only if the frequency lock is successfully acquired.

This behavior seems consistent with other demodulator drivers (e.g. stv090x,
hd29l2, stv0900, stb0899, mb86a16).

This patch has been successfully tested with DVB-T and DVB-T2 signals,
on both PC and the mipsel STB running Enigma2.
No apparent side effect has been observed on PC applications like Kaffeine.
DVB-C is not available in my country so it's not tested.

Signed-off-by: Gianluca Gennari <gennarone@gmail.com>
Acked-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>