xenfb: use the correct condition to avoid excessive looping

In commit ac0487e1 ("xenfb.c: avoid expensive loops when prod <=
out_cons"), ">=" was used. In fact, a full ring is a legit state.
Correct the test to use ">".

Reported-by: "Hao, Xudong" <xudong.hao@intel.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Tested-by: "Hao, Xudong" <xudong.hao@intel.com>
Acked-by: Anthony Perard <anthony.perard@citrix.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
