wimax/i2400m: fix incorrect return -ESHUTDOWN when there is no Tx buffer available

i2400m_tx() routine was returning -ESHUTDOWN even when there was no Tx buffer
available. This patch fixes the i2400m_tx() to return -ESHUTDOWN only when
the device is down(i2400m->tx_buf is NULL) and also to return -ENOSPC
when there is no Tx buffer. Error seen in the kernel log.
kernel: i2400m_sdio mmc0:0001:1: can't send message 0x5606: -108
kernel: i2400m_sdio mmc0:0001:1: Failed to issue 'Enter power save'command: -108

Signed-off-by: Prasanna S.Panchamukhi <prasannax.s.panchamukhi@intel.com>

drivers/net/wimax/i2400m/tx.c

@@ -643,9 +643,11 @@ int i2400m_tx(struct i2400m *i2400m, const void *buf, size_t buf_len,
 	 * current one is out of payload slots or we have a singleton,
 	 * close it and start a new one */
 	spin_lock_irqsave(&i2400m->tx_lock, flags);
-	result = -ESHUTDOWN;
-	if (i2400m->tx_buf == NULL)
+	/* If tx_buf is NULL, device is shutdown */
+	if (i2400m->tx_buf == NULL) {
+		result = -ESHUTDOWN;
 		goto error_tx_new;
+	}
 try_new:
 	if (unlikely(i2400m->tx_msg == NULL))
 		i2400m_tx_new(i2400m);