soc: bcm: brcmstb: remove unused variable 'brcmstb_machine_match'

Fix the following clang warning:

drivers/soc/bcm/brcmstb/common.c:17:34: warning: unused variable
'brcmstb_machine_match' [-Wunused-const-variable].

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>