serial: bfin_sport_uart: rename early platform driver class string

Clarifies command line set up for devices between consoles and early
devices.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>