staging: vchiq_arm: Remove unused variable

This warning appears with GCC 6.4.0 from toolchains.bootlin.com:

../drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c: In function ‘vchiq_open’:
../drivers/staging/vc04_services/interface/vchiq_arm/vchiq_arm.c:1735:7: warning: unused variable ‘ret’ [-Wunused-variable]
   int ret;
       ^~~

This variable's usage was removed by commit 3c980263 ("staging:
vchiq_arm: Make debugfs failure non-fatal"), making it useless.

Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>