Staging: zram: simplify zram_make_request

zram_read() and zram_write() always return zero, so make them return
void to simplify the code.

Signed-off-by: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: Jerome Marchand <jmarchan@redhat.com>
Acked-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>