niu: Change niu_rbr_fill() to use unlikely() to check niu_rbr_add_page() return value

Change niu_rbr_fill() to use unlikely() to check niu_rbr_add_page() return
value to be consistent with the rest of the checks after niu_rbr_add_page()
calls in this file.

Signed-off-by: Shuah Khan <shuah.khan@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>