brcmfmac: enlarge buffer size of caps to 512 bytes

The buffer size of return of cap iovar is greater than 256 bytes in some
firmwares. For instance, the return size of cap iovar is 271 bytes in 4373
13.10.246.79 firmare. It makes feature capability parsing failed because
caps buffer is default value.
So we enlarge caps buffer size to 512 bytes and add the error print for
cap iovar error.

Signed-off-by: Wright Feng <wright.feng@cypress.com>
Acked-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>