Input: egalax_ts_serial - fix potential NULL dereference on error

We didn't check input_allocate_device() for failures so it could lead to
a NULL deref.

Fixes: 6b0f8f9c ('Input: add eGalaxTouch serial touchscreen driver')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>