[spi] reformat message and ar5315_spi minor fix

This commit corrects print of nomatch (newline was too early) and fix
unit number for new child in ar5315_spi (was 0, now is -1 to calculate it
according to actual system state)

Submitted by: Hiroki Mori <yamori813@yahoo.co.jp>
Reviewed by: ray, loos, mizhka
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D8749