ath6kl: fix uninitialized warning in ath6kl_process_uapsdq()

Before I commited patch c1762a3fe ("ath6kl: Add support for uAPSD") I
did a minor change how up variable is initialised in
ath6kl_process_uapsdq(). But I was sloppy and caused this compiler
warning:

txrx.c:88:5: warning: 'up' may be used uninitialized in this function

Revert my change to fix the warning.

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