selftests: tpm: Use 'test -e' instead of 'test -f'

'test -f' is suitable only for *regular* files. Use 'test -e' instead.

Cc: Nikita Sobolev <Nikita.Sobolev@synopsys.com>
Cc: linux-integrity@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org
Fixes: 5627f9cf

 ("Kernel selftests: Add check if TPM devices are supported")
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>