video: stm32: stm32_ltdc: fix the check of return value of clk_set_rate()

The clk_set_rate() function returns rate as an 'ulong' not
an 'int' and rate > 0 by default.

This patch avoids to display the associated warning when
the set rate function returns the new frequency.

Fixes: aeaf3306 ("video: stm32: stm32_ltdc: add bridge to display controller")
Signed-off-by: Gabriel Fernandez <gabriel.fernandez@foss.st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>