spl: Correct SPL_SIZE_LIMIT Kconfig option

When introduced this limit was an int but was then changed to hex
without noting as much in the prompt nor changing existing users.  Put
this back to an int.

Reported-by: Fabio Estevam <festevam@gmail.com>
Tested-by: Fabio Estevam <festevam@gmail.com>
Fixes: 2577015d ("spl: add overall SPL size check")
Signed-off-by: Tom Rini <trini@konsulko.com>