Input: mxs-lradc - remove redundant assignment to pointer input

The pointer 'input' is being initialized with ts->ts_input and this
value is not being read as it is updated a few lines later with the
return value from the call to devm_input_allocate_device. Remove the
redundant initialization assignment.  Cleans up clang warning:

drivers/input/touchscreen/mxs-lradc-ts.c:587:20: warning: Value Xi
stored to 'input' during its initialization is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>