drm/radeon: fix fb_div check in ni_init_smc_spll_table()

clk_s is checked twice in a row in ni_init_smc_spll_table().
fb_div should be checked instead.

Fixes: 69e0b57a ("drm/radeon/kms: add dpm support for cayman (v5)")
Cc: stable@vger.kernel.org
Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>