zram: convert remaining CLASS_ATTR() to CLASS_ATTR_RO()

I missed converting the last zram attribute to CLASS_ATTR_RO() after
removing CLASS_ATTR() from the kernel, causing a build breakage.  This
patch fixes that problem.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>