cmd_sf.c: fix printf() length modifier

cmd_sf.c: fix printf() length modifier

size_t is not always 'unsigned int', use corret length modifer.

This patch fixes following warning:

---8<---
cmd_sf.c: In function 'spi_flash_update_block':
cmd_sf.c:130: warning: format '%#x' expects type 'unsigend int', but argument 4 has type 'size_t'
cmd_sf.c:135: warning: format '%x' expects type 'unsigned int', but argument 3 has type 'size_t'
--->8---

Signed-off-by: Andreas Bießmann <[email protected]>
cc: Mike Frysinger <[email protected]>
cc: Thomas Chou <[email protected]>
Acked-by: Mike Frysinger <[email protected]>