spi/ep93xx: Fix format specifier warning

This patch fixes the following sparse warning by changing the
the format specifier for size_t to %zu.

drivers/spi/spi-ep93xx.c:512:3: warning:
	format ‘%d’ expects argument of	type ‘int’,
	but argument 3 has type ‘size_t’ [-Wformat]

Signed-off-by: Emil Goode <emilgoode@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>