p54: return NETDEV_TX_OK in p54_tx and fix sparse warnings

p54: return NETDEV_TX_OK in p54_tx and fix sparse warnings

This patch addresses all recent comments from Johannes Berg:

1st: (reference http://marc.info/?l=linux-wireless&m=123124685019631 )
>First off: all those should return NETDEV_TX_OK/BUSY.
>iwl-agn: returns 0 (== NETDEV_TX_OK, but still should be changed)
>[...]
>p54: same (some paths)

2nd:
> due to your PS patch ("p54: power save management"), please run sparse:
> make C=2 CF=-D__CHECK_ENDIAN__ M=...
> +drivers/net/wireless/p54/p54common.c:1753:8: warning: incorrect type in assignment (different base types)
> +drivers/net/wireless/p54/p54common.c:1769:29: warning: incorrect type in assignment (different base types)
The cpu_to_le16 ended up in the wrong line... Sorry!

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>