mfd: ti_am335x_tscadc: Remove redundant assignment to node

Node is being initialized a value that is never read, it is
being written over a few statements into the function with
the return value from call to of_get_child_by_name. Hence
this initialization can be removed. Cleans up clang warning:

drivers/mfd/ti_am335x_tscadc.c:127:22: warning: Value stored
to 'node' during its initialization is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>