qcacld-3.0: fix code defects for spectral callback registering

Declaring variable spectral_cb without initializer in
sme_register_spectral_cb, spectral_cb.vdev_get_chan_freq_seq2
will be used uninitialized.
To fix it, initialize spectral_cb when declaring.

CRs-Fixed: 2751599
Change-Id: I2c5cd5a85b54dcbda3059bfd528cd18626b6b316