brcmfmac: sdio: remove unused variable retry_limit

retry_limit has never been used during the life of this driver, so
we may as well remove it as it is redundant.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Julian Calaby <julian.calaby@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>