brcmfmac: cleanup brcmf_cfg80211_escan() function

The function brcmf_cfg80211_escan() was always called with a non-null
request parameter and null pointer for this_ssid parameter. Clean up
the function removing the dead code path.

Reviewed-by: Hante Meuleman <hante.meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieter-paul.giesberts@broadcom.com>
Reviewed-by: Franky Lin <franky.lin@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>