move duplicated definitions of the line discipline numbers N_* in asm-blackfin/termios.h

The definitions of the line discipline numbers N_* have been moved
from asm-*/termios.h to linux/tty.h, but the Blackfin architecture
has somehow evaded that move. Bring it in line with the others.

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>

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