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 <biessmann@corscience.de>
cc: Mike Frysinger <vapier@gentoo.org>
cc: Thomas Chou <thomas@wytron.com.tw>
Acked-by: Mike Frysinger <vapier@gentoo.org>