ACPI: ibm-acpi: clean up fan_read

This patch cleans up fan_read so that it is much easier to read and
extend.

The patch fixes the userspace ABI to return "status: not supported" (like
all other ibm-acpi functions) when neither fan status or fan control are
possible.

It also fixes the userspace ABI to return EIO if ACPI access to the EC
fails, instead of returning "status: unreadable" or "speed: unreadable".

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>