spl: Correct call to spl_common_init() with SPL_STACK_R_MALLOC_SIMPLE_LEN

Calls to IS_ENABLED() on a non-y/n option will always be false, even
when set.  We can correct this by adding a new bool value that is set
based on the conditions required for SPL_STACK_R_MALLOC_SIMPLE_LEN to be
set instead.

Fixes: 340f418a ("spl: Add spl_early_init()")
Reported-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
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Changes in v2:
- Fix thinko pointed out by Lokesh