ath6kl: add prefix parameter to ath6kl_dbg_dump()

Makes it easier to recognise longs dumps.

Obligatory screenshot using "rx" prefix:

ath6kl: ath6kl_rx
rx 00000000: 10 10 00 00 00 00 08 30 00 00 00 00 00 00 f9 0b  .......0........
rx 00000010: 2c 44 08 30 00 00 f9 0b 0c a4 02 00 00 00 73 d2  ,D.0..........s.
rx 00000020: 94 00 f9 0b 04 8c 01 00 02 00 07 02 02 00 f9 0b  ................

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>