msm_serial_hs: Don't destroy mutex from msm_hs_shutdown()

mutex is used uninitialized while unloading N_TTY line discipline
and installing SMUX Line discipline as msm_hs_shutdown() of HSUART
driver is called which is destroying this used mutex. Hence while
SMUX Line discipline is used, and clock off API is called, mutex
slowpath warning is seen. Destroying of mutex shouldn't be done from
msm_hs_shutdown() as initizalition and destroying of the mutex
is done from probe() and remove() of HSUART driver. Hence don't
destroy mutex from msm_hs_shutdown() to fix the issue.

CRs-Fixed: 391551
Change-Id: I74ac1372405e1b893d658ee03e9955858c9e782f
Signed-off-by: Mayank Rana <mrana@codeaurora.org>