brcmfmac: make pointer type constant in brcmf_set_management_ie()

The vendor ie buffer passed to brcmf_set_management_ie() is not modified
and the caller always provided a constant buffer, which needed a cast.
Better making the buffer parameter of the function constant so the
casts can be removed.

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: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>