drivers: serial: uart_stm32: fix IRQ lock not released on error path

In uart_stm32_async_rx_buf_rsp(), when the provided RX buffer is not
located in non-cacheable memory, the function would return -EFAULT
while still holding the IRQ lock acquired at the start of the critical
section. This causes a system deadlock as interrupts remain disabled
indefinitely.

The nocache validation check has been moved outside the critical
section to ensure proper error handling. The check is performed before
acquiring irq_lock(), maintaining the same validation behavior while
preventing the lock from being held during the early error return.

Fixes a bug where async UART RX operations with improperly
placed buffers would hang the system.

Signed-off-by: Miika Karanki <miika.karanki@vaisala.com>