soc: qcom: glink_spi_xprt: Add check for SSR during RX polling

The glink ssr function blocks until all RCU references are returned.
The polling mode in the rx worker thread delays SSR shutdown
because it holds the RCU reference until polling is finished.
Add check for ssr in polling loop.

CRs-Fixed: 1111153
Change-Id: Ie51e6ed9989411f28639857b40c5603b5e1064c3
Signed-off-by: Chris Lew <clew@codeaurora.org>