dw9714l: fix uninitialized variable

Fix build:
dw9714l.c: In function 'dw9714l_ioctl':
dw9714l.c:115:6: error: 'ret' may be used uninitialized in this function [-Werror=maybe-uninitialized]

Change-Id: I41e6d4879ddec2cb0d47b83ef67cb38bc00e53e9