spl: fix debug print in spl_common_init()

spl_common_init() debug-prints "spl_early_init()\n" but it is
called both from spl_early_init() and spl_init().

Fix this by moving the debug() statement to the calling functions
which now print their name.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>