Fix reading of BER, SNR and strength values.

According to Linux DVB docs FE_READ_SIGNAL_STRENGTH and FE_READ_SNR
have int16_t parameter, not int32_t.

Also FE_READ_BER returns uint32_t, not int32_t.

Using the wrong types caused dvblastctl to report absurd values
for strength and snr.

Reported by: Ibrahim Tachijian <barhom@gmail.com>

Signed-off-by: Georgi Chorbadzhiyski <gf@unixsol.org>