tpm: Change response length of tpm2_get_capability()

For implementing the EFI_TCG2_PROTOCOL we need the count field returned by
the TPM when reading capabilities via tpm2_get_capability().

Adjust the implementation of the 'tpm2 get_capability' command accordingly.

Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>