tehuti: Firmware filename is tehuti/bdx.bin

My conversion of tehuti to use request_firmware() was confused about
the filename of the firmware blob.  Change the driver to match the
blob.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: David S. Miller <davem@davemloft.net>