i2c: brcmstb: fix support for DSL and CM variants

DSL and CM (Cable Modem) support 8 B max transfer size and have a custom
DT binding for that reason. This driver was checking for a wrong
"compatible" however which resulted in an incorrect setup.

Fixes: e2e5a2c6

 ("i2c: brcmstb: Adding support for CM and DSL SoCs")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>