rt2x00: rt2800usb: fix status register reread logic

Another good catch from Jakub Kicinski. This patch fixes my
recent commit: ed61e2b0
"rt2x00: rt2800usb: rework txdone code"

We should reread status register only when nobody else start already
reading status i.e. test_and_set_bit(TX_STATUS_READING, flags) return 0.

Reported-by: Jakub Kicinski <moorray@wp.pl>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>