usb: gadget: f_fs: remove custom printk() wrappers

This commit removes custom printk() wrappers from the f_fs.c
file.  They served little purpose above what pr_*() family of
macros provides.  Only FVDBG() has been left but renamed to
pr_vdebug() to match other uses.
Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>