xtensa: One function call less in bootmem_init()

Avoid an extra function call by using a ternary operator instead of
a conditional statement for a setting selection.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Message-Id: <495c9f2e-7880-ee9a-5c61-eee598bb24c2@web.de>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>