tty: baudrate: Explicit usage of B0 for encoding input baudrate

tty: baudrate: Explicit usage of B0 for encoding input baudrate

Function tty_termios_input_baud_rate() checks IBSHIFT & CBAUD against B0
constant to determinate if input speed equals to output speed. So do same
B0 check also in tty_termios_encode_baud_rate() function to make these two
functions compatible.

Currently macro B0 is defined to 0 so there is no functional change.

This change just make code more explicit to show that Bnnn constants are
stored in CBAUD.

Signed-off-by: Pali Rohár <pali@kernel.org>
Link: https://lore.kernel.org/r/20210927133516.8671-1-pali@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>