soc: spcom: fix sleep only on pending send command

This change fixes unconditional sleep after sending data to RPMSG
peer, need to sleep only after not successful rpmsg_trysend.

Change-Id: Iaa8adb43f397863b50200c4b38d9d05b4b99e89f
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>