staging: vchiq_2835_arm: Remove unnecessary assignment to slot_mem_size

The variable slot_mem_size is assigned a value which is never used.
This issue has been found by CppCheck.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>