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

When building for ARM64 the following build warnings are seen:

drivers/staging/fbtft/fbtft-io.c: In function 'fbtft_write_spi_emulate_9':
drivers/staging/fbtft/fbtft-io.c:63:4: warning: format '%d' expects argument of type 'int', but argument 4 has type 'size_t' [-Wformat=]

drivers/staging/fbtft/fbtft-io.c: In function 'fbtft_read_spi':
drivers/staging/fbtft/fbtft-io.c:110:5: warning: format '%d' expects argument of type 'int', but argument 4 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>