opal_console: fix serial console output corruption on powerpc64

Adds OPAL_CONSOLE_WRITE error handling and implements a call to
OPAL_CONSOLE_WRITE_BUFFER_SPACE to verify if there's enough space
before writing to console.

This fixes serial port output getting corrupted on fast writes, like
on "dmesg" output.

Tested on Raptor Blackbird running powerpc64 BE kernel

Reviewed by: luporl
Sponsored by: Eldorado Reserach Institute (eldorado.org.br)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D29063