ath6kl: add support for Dell Wireless 1537

This is a Qualcomm Atheros AR6004X with an sdio ID of 0x19 and hardware ID of
0271:0419. Tested on a Dell Venue 11 Pro 7130 with a self compiled kernel.

Signed-off-by: Guy Chronister <guylovesbritt@gmail.com>
[kvalo@codeaurora.org: cleanup commit log]
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>