mfd: Use correct variable name for tps65910 regmap config

This was the copy-paste issue in reg cache support code where
variable name for regmap config was not really starting from
the device name, it was starting from some other device name.
Fixing this so that variable name contains actual device name.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>