ath10k: pci: remove unnecessary casts

Fixes checkpatch warnings:

drivers/net/wireless/ath/ath10k/pci.c:926: unnecessary cast may hide bugs, see http://c-faq.com/malloc/mallocnocast.html
drivers/net/wireless/ath/ath10k/pci.c:1072: unnecessary cast may hide bugs, see http://c-faq.com/malloc/mallocnocast.html

While at it, also remove unnecessary initialisation of data_buf variable in both cases.

Signed-off-by: Kalle Valo <kvalo@codeaurora.org>