staging: fbtft: fbtft-core: Use '%zu' to print 'size_t' format

When building for ARM64 the following build warning is seen:

drivers/staging/fbtft/fbtft-core.c:1004:4: warning: format '%d' expects argument of type 'int', but argument 3 has type 'size_t' [-Wformat=]

Use '%zu' to print 'size_t' format.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>