video: mxsfb: fix clk_get_by_name() return value check

If clk_get_by_name() returns 0 it means it executed successfully while now
we consider it as an error. So let's check if return value is negative to
be an error. Otherwise this prevents "axi" and "disp_axi" to be enabled.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>