acpi_gpiobus: clarify error message

This message was identical to another a few lines up, making it hard to
tell where the error occurred.

Reviewed-by: Ed Maste <emaste@FreeBSD.org>
Reviewed-by: Warner Losh <imp@FreeBSD.org>
Pull-request: https://github.com/freebsd/freebsd-src/pull/1667
Signed-off-by: Ahmad Khalifa <ahmadkhalifa570@gmail.com>