tty: support CIBAUD without BOTHER

Since commit edc6afc5 ("[PATCH] tty: switch to ktermios and new
framework") arbitrary baud rates can be requested using BOTHER and input
rates can be requested using the termios CIBAUD bits (CBAUD shifted
IBSHIFT bits).

This functionality has been conditionally compiled depending on whether
an architecture defines BOTHER and IBSHIFT respectively, but would in
fact fail to compile unless both symbols were defined due to cross
dependencies.

Relax the IBSHIFT => BOTHER dependency so that an architecture could
theoretically support CIBAUD without the Linux-specific BOTHER, while
hopefully making the current conditional-compilation directives a bit
less confusing.

Note that the long-term goal is still to have all architectures support
both features, so an alternative could just be to have the lot depend on
BOTHER.

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>