tests: drivers: flash: Improve compatibility with 64-bit platforms

Change variables in test_cb_data_type from uint32_t to size_t to
match the return type of flash_get_page_count() and adjust
format specifiers accordingly. This resolves warnings when
compiling for 64-bit platforms.

Signed-off-by: Tim Pambor <tim.pambor@codewrights.de>