w83627ehf: Fix the detection of fan5

Fix the detection of fan5 and preserve the bit between the
register writes, because the bit is write only.

Signed-off-by: Rudolf Marek <r.marek@assembler.cz>
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>