efi: efivars: add efivars printk prefix

Add an 'efivars: ' printk prefix to make the log entries stand out more,
for example:

	efivars: Registered efivars operations

While at it, change the sole remaining direct printk() call to pr_err().

Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>