tpm(4): Fix GCC build after r342084 (TPM 2.0 driver commit)

Move static variable definition (cdevsw) to a more conventional location
(the C file it is used in), rather than a header.

This fixes the GCC warning, -Wunused-variable ("defined but not used") when
the tpm20.h header is included in files other than tpm20.c (e.g.,
tpm_tis.c).

X-MFC-with: r342084
Sponsored by: Dell EMC Isilon