ssb: Drop legacy header include

The SSB header only uses the legacy <linux/gpio.h> header to get
struct gpio_chip so inluce <linux/gpio/driver.h> which is the right
include to deal with gpio_chip.

Cc: Michael Buesch <m@bues.ch>
Cc: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210821235800.138817-1-linus.walleij@linaro.org

include/linux/ssb/ssb.h

@@ -7,7 +7,7 @@
 #include <linux/types.h>
 #include <linux/spinlock.h>
 #include <linux/pci.h>
-#include <linux/gpio.h>
+#include <linux/gpio/driver.h>
 #include <linux/mod_devicetable.h>
 #include <linux/dma-mapping.h>
 #include <linux/platform_device.h>