staging: typec: tcpm: make function tcpm_get_pwr_opmode

The function pointer tcpm_get_pwr_opmode is local to the source and does
not need to be in global scope, so make it static.

Cleans up sparse warning:
symbol 'tcpm_get_pwr_opmode' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>