brcmfmac: get rid of brcmf_fw_get_full_name()

The function was pretty minimal and now it is called only from one
place so just get rid of it.

Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>