staging: winbond: use dev_err() instead of printk()

For obvious error messages, use dev_err() in order to provide userspace
with more useful information and use the common kernel coding style.

Signed-off-by: Martin Kepplinger <martink@posteo.de>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>