drm/bridge: analogix_dp: make analogix_dp_start_aux_transaction() static

This eliminates the following sparse warning:

drivers/gpu/drm/bridge/analogix/analogix_dp_reg.c:527:5: warning: symbol
'analogix_dp_start_aux_transaction' was not declared. Should it be
static?
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200912033843.143240-1-yanaijie@huawei.com

drivers/gpu/drm/bridge/analogix/analogix_dp_reg.c

@@ -524,7 +524,7 @@ void analogix_dp_enable_sw_function(struct analogix_dp_device *dp)
 	writel(reg, dp->reg_base + ANALOGIX_DP_FUNC_EN_1);
 }
 
-int analogix_dp_start_aux_transaction(struct analogix_dp_device *dp)
+static int analogix_dp_start_aux_transaction(struct analogix_dp_device *dp)
 {
 	int reg;
 	int retval = 0;