drivers: rmnet_shs: Fix shs_boost_wq memleak

Previously when SSR occured a new shs_freq_wq would be allocated
if shs initialized.

This change destroys the workqueue on de-initialization so that
allocating isn't resulting in a memleak.

Change-Id: I6282890f9fd3c6c562903ff7b64b014ad2658f38
Acked-by: Raul Martinez <mraul@qti.qualcomm.com>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>