brcmfmac: make non-global functions static

Fix sparse warning:
drivers/net/wireless/broadcom/brcm80211/brcmfmac/p2p.c:2206:5:
	warning: symbol 'brcmf_p2p_get_conn_idx' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Reviewed-by: Wright Feng <wright.feng@cypress.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200508013249.95196-1-chenzhou10@huawei.com