prism54: isl_ioctl: Fix one function header and demote another

Fixes the following W=1 kernel build warning(s):

 from drivers/net/wireless/intersil/prism54/isl_ioctl.c:22:
 inlined from ‘prism54_get_name’ at drivers/net/wireless/intersil/prism54/isl_ioctl.c:283:2:
 drivers/net/wireless/intersil/prism54/isl_ioctl.c:68: warning: Function parameter or member 'priv' not described in 'prism54_mib_mode_helper'
 drivers/net/wireless/intersil/prism54/isl_ioctl.c:68: warning: Excess function parameter 'mib' description in 'prism54_mib_mode_helper'
 drivers/net/wireless/intersil/prism54/isl_ioctl.c:127: warning: Function parameter or member 'priv' not described in 'prism54_mib_init'

Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Aurelien Alleaume <slts@free.fr>
Cc: Valerio Riedel <hvr@gnu.org>
Cc: "Luis R. Rodriguez" <mcgrof@ruslug.rutgers.edu>
Cc: linux-wireless@vger.kernel.org
Cc: netdev@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201102112410.1049272-15-lee.jones@linaro.org