wl18xx: use %zu for size_t arguments in printk calls

After 934b9d1e (wl18xx: avoid some -Wformat warnings) there was still
a warning with (at least) ARM gcc version 4.4.1:

drivers/net/wireless/ti/wl18xx/main.c: In function 'wl18xx_conf_init':
drivers/net/wireless/ti/wl18xx/main.c:1026: warning: format '%ld' expects type 'long int', but argument 2 has type 'unsigned int'

Fix this by using %zu for the both formats, since the fw->size and the
macro (derived from sizeof()) are size_t.

Signed-off-by: Luciano Coelho <coelho@ti.com>