char: xillybus: Fix spelling mistake and comment

This patch fixes two minor issues:

(1) An inaccurate comment
(2) A spelling mistake in dev_err message ("upgarde" -> "upgrade")

Reported-by: Joe Perches <joe@perches.com>
Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>