staging: vchiq_arm: Drop unnecessary NULL check

Looking at the handling of service instance within the VCHIQ
driver shows that it's not possible that service_callback is
called with instance is a NULL pointer. So drop the unnecessary
NULL check and fix:

vchiq_arm.c:1109 service_callback() warn:
  variable dereferenced before check 'instance' (see line 1091)

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/linux-arm-kernel/202404230315.vx7ESZ3r-lkp@intel.com/


Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Reviewed-by: Umang Jain <umang.jain@ideasonboard.com>
Link: https://lore.kernel.org/r/20240425165852.6681-2-wahrenst@gmx.net


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>