mfd: Fix twl-core oops while calling twl_i2c_* for unbound driver

commit 8653be1a upstream.

Check inuse variable before trying to access twl_map to prevent
dereferencing of uninitialized variable.

Signed-off-by: Ilya Yanok <yanok@emcraft.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>