rt2x00: fix typo in rt2800.h

Fix a typo in a comment in rt2800.h. Instead of replacing the wrong
hexvalue (0x171c) with the correct one (0x1718) just use the appropriate
readable define.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>