acpi(4): Fix a bug in acpi_cpu_cstate.c (we have to write, and not to read).

acpi(4): Fix a bug in acpi_cpu_cstate.c (we have to write, and not to read).

Introduced with 10f976749fd9ad2e8642ea80ce533f7416910a65. The commit message
said "Sync ACPI with FreeBSD 7.2", even though FreeBSD 7.2 doesn't seem to
have this code at all, so I'm not sure about what the idea behind that
change was. I'm guessing it is a typo, since newer FreeBSDs call
AcpiWriteBitRegister() here too.

Reported-by: Andrea Magliano <masterblaster@tiscali.it>