tpm_registers: check if_stop has been initialized before calling it
tpm_registers: check if_stop has been initialized before calling it

It's possible tpm_stop will be called before the interface has been
registered. Don't call if_stop unless it has been initialized.

BUG=none
BRANCH=cr50
TEST=none

Change-Id: I45b6d11553ce0eda5b097184a5be81f8b79726aa
Signed-off-by: Mary Ruthven <81380f3403fed74252e92df3ed012a1819ec6296@google.com>
Reviewed-on: https://chromium-review.googlesource.com/1480710
Commit-Ready: Mary Ruthven <81380f3403fed74252e92df3ed012a1819ec6296@chromium.org>
Tested-by: Mary Ruthven <81380f3403fed74252e92df3ed012a1819ec6296@chromium.org>
Reviewed-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>
Reviewed-by: Keith Short <f6d578f97033c25f1206cb57d52f358974c82954@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec