tpm(4): Fix 'go ready' in TPM 2.0 TIS driver

tpmtis_go_ready() read the value of the TPM_STS register, ORed
TPM_STS_CMD_READY with it, and wrote it back. However, the TPM Profile
(PTP) specification states that only one bit in the write request value may
be set to 1, or else the entire write request is ignored.

Fix by just writing TPM_STS_CMD_READY.

Similarly, remove the call which clears the TPM_STS_CMD_READY flag in the
same function. It was being ignored for the same reason.

Submitted by: Darrick Lew <darrick.freebsd AT gmail.com>
Reviewed by: vangyzen, myself
MFC after: if you care about stable, you might want to do so
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D23081