rpm-smd: Remove redundant spinlocks which are not required

As rpmsg takes care of locking when sending data, this patch
removes the redundant locking by removing the smd_write spinlock.

Also this patch reverts the 'commit 808861b8 ("drivers: rpmsg:
fix to avoid dump stack in rpm-smd driver")' which is not required
after removing extra spinlocks.

It removes the unsed smd_read spinlock aswell.

Change-Id: I7b15d19add2c4da876aa127be233331b58d3c8ae
Signed-off-by: Raghavendra Kakarla <rkakarla@codeaurora.org>