staging: ccree: simplify buf mgr using local vars

Make the code more readable by using a local variables
for commonly use expressions in the buffer manager part
of the driver.

Signed-off-by: Gilad Ben-Yossef <gilad@benyossef.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>