1) Change csr_get_cfg_valid_channels's code to make it
use channel frequencies directly to calculate a
valid channel list.
2) Add kernel-doc for csr_get_cfg_valid_channels.
3) Remove the definition, declaration and usage of
csr_get_cfg_valid_freq as csr_get_cfg_valid_channels
does exactly the same job.
Change-Id: I6745cb2220bf9387c64ac8c271297cb89f821ab3
CRs-Fixed: 2560091
2bab3a5b7c