staging: ozwpan: Remove redundant null check before kfree in ozpd.c

kfree on NULL pointer is a no-op.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Rupesh Gujare <rgujare@ozmodevices.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>