Fixes a warning while compiling zynq_mkimage.c
scripts/zynq_mkimage.c:312:2: warning: ignoring return value of ‘fread’, declared with attribute warn_unused_result [-Wunused-result]
fread returns the number bytes read, if it is not equal to st_size some error has happend

Signed-off-by: Michael Graichen <michael.graichen@hotmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>