Update the APIs mentioned below, to handle invalid channel number and
freq value of less than or equal to 0:
1) wlan_reg_chan_band_to_freq
2) wlan_reg_freq_to_chan
3) wlan_reg_chan_to_freq
4) wlan_reg_legacy_chan_to_freq
Change-Id: I122658c974c550ff16bcfe1b664799a609c7ffcf
CRs-Fixed: 2550286