gpu: host1x: clk_round_rate() can return a zero upon error

Treat both negative and zero return values from clk_round_rate() as
errors. This is needed since subsequent patches will convert
clk_round_rate()'s return value to be an unsigned type, rather than a
signed type, since some clock sources can generate rates higher than
(2^31)-1 Hz.

Eventually, when calling clk_round_rate(), only a return value of zero
will be considered a error. All other values will be considered valid
rates. The comparison against values less than 0 is kept to preserve
the correct behavior in the meantime.

Signed-off-by: Paul Walmsley <[e-mail]>
Cc: Mikko Perttunen <[e-mail]>
Cc: Arto Merilainen <[e-mail]>
Cc: Thierry Reding <[e-mail]>
Cc: Terje Bergström <[e-mail]>
Signed-off-by: Thierry Reding <[e-mail]>

drivers/gpu/drm/tegra/hdmi.c

···
 	parent = clk_get_parent(hdmi->clk_parent);
 
 	err = clk_round_rate(parent, pclk * 4);
-	if (err < 0)
+	if (err <= 0)
 		*status = MODE_NOCLOCK;
 	else
 		*status = MODE_OK;