drivers/tty/serial/crisv10.c: remove dead #ifdef block

ETRAX_EXTERN_PB6CLK_ENABLED is not defined in Kconfig.  The affected
 #ifdef block has not been compiled for years, so remove it entirely.

Signed-off-by: Valentin Rothberg <valentinrothberg@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>