ath11k: support loading ELF board files

The QCA6390 board I have, model 8291M-PR comes with an ELF board file.  To get
this to at least somewhat work, I renamed bdwlan.e04 to 'board.bin' and then
added this patch to check for ELF magic string in the beginning of the file.
If that is found, use type ELF.  After this the driver loads.

Tested-on: QCA6390 hw2.0 PCI WLAN.HST.1.0.1-01740-QCAHSTSWPLZ_V2_TO_X86-1

Signed-off-by: Ben Greear <greearb@candelatech.com>
[kvalo@codeaurora.org: use elf.h, minor cleanup]
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1601399736-3210-2-git-send-email-kvalo@codeaurora.org