brcmfmac: increase buffer for obtaining firmware capabilities

When obtaining the firmware capability a buffer is provided of 512
bytes. However, if all features in firmware are supported the buffer
needs to be 565 bytes as otherwise truncated information is retrieved
from firmware. Increasing the buffer to 768 bytes on stack.

Reviewed-by: Hante Meuleman <hante.meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieter-paul.giesberts@broadcom.com>
Reviewed-by: Franky Lin <franky.lin@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>