brcmfmac: get rid of brcmf_cfg80211_set_channel() function

brcmfmac: get rid of brcmf_cfg80211_set_channel() function

The function does not provide any additional functionality and is
used only once so just get rid of it.

Reviewed-by: Daniel (Deognyoun) Kim <dekim@broadcom.com>
Reviewed-by: Franky Lin <frankyl@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>