rt2x00: Fix txdone_entry_desc_flags

txdone_entry_desc_flags is used with __set_bit and test_bit which
bit-shift the values, so don't bit-shift the flags in the enum.
Also make sure flags are initialized before being used.

Signed-off-by: Jochen Friedrich <jochen@scram.de>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>