libata-transport.c: local functions should not be exposed globally

Functions not referenced outside of a source file should be marked
static to prevent it from being exposed globally.

This quiets the sparse warnings:

warning: symbol 'ata_is_port' was not declared. Should it be static?
warning: symbol 'ata_is_link' was not declared. Should it be static?
warning: symbol 'ata_is_ata_dev' was not declared. Should it be static?

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>