rtw88: fix spelling mistake "fimrware" -> "firmware"

There are spelling mistakes in two rtw_err error messages. Fix them.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200424084733.7716-1-colin.king@canonical.com