cmd/ubi.c: Fix format warning

On 64bit platforms we would otherwise see:
../cmd/ubi.c: In function 'ubi_volume_read':
../cmd/ubi.c:359:16: warning: format '%u' expects argument of type 'unsigned int', but argument 2 has type 'size_t {aka long unsigned int}' [-Wformat=]

Fixes: 68c7025d ("cmd: ubi: print load size after establishing volume size")
Signed-off-by: Tom Rini <trini@konsulko.com>