usb: host: xhci-debugfs: Use 'gnu_printf' format notation

Fixes the following W=1 kernel build warning(s):

 drivers/usb/host/xhci-debugfs.c:128:2: warning: function ‘xhci_debugfs_regset’ might be a candidate for ‘gnu_printf’ format attribute [-Wsuggest-attribute=format]
 128 | vsnprintf(rgs->name, sizeof(rgs->name), fmt, args);
 | ^~~~~~~~~

Cc: Mathias Nyman <mathias.nyman@intel.com>
Cc: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200703174148.2749969-19-lee.jones@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci-debugfs.c

@@ -110,6 +110,7 @@ static void xhci_debugfs_free_regset(struct xhci_regset *regset)
 	kfree(regset);
 }
 
+__printf(6, 7)
 static void xhci_debugfs_regset(struct xhci_hcd *xhci, u32 base,
 				const struct debugfs_reg32 *regs,
 				size_t nregs, struct dentry *parent,