usb: gadget: g_fs: Fix compilation warning

This commit fixes warning in f_fs.c introduced by "usb:
gadget: f_fs: remove custom printk() wrappers":

In file included from drivers/usb/gadget/g_ffs.c:64:
drivers/usb/gadget/f_fs.c:30:1: warning: "pr_fmt" redefined

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>