brcmfmac: move brcmf_c_set_joinpref_default declaration to common.h

Function brcmf_c_set_joinpref_default is in common.c, so move it to the
related header.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>