staging: comedi: drivers: s626.c - fixed checkpatch issue about data type

staging: comedi: drivers: s626.c - fixed the following checkpatch issue:
CHECK: Prefer kernel type 's16' over 'int16_t'
#1939: FILE: drivers/staging/comedi/drivers/s626.c:1939:
+               int16_t dacdata = (int16_t)data[i];

Signed-off-by: Andrea della Porta <sfaragnaus@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>