input: synaptics_dsx: Correct error path for write_buf memory allocation

Correct the error path for the memory allocation of write_buf variable
in Synaptics DSX touch driver. Properly free the memory associated to it.

Change-Id: Id66a123332d8a88519119e42d21d338abe550dbf
Signed-off-by: Shantanu Jain <shjain@codeaurora.org>