rmnet_smux: Fix duplicate packet send

If the high watermark is exceeded, then msm_smux_write() will return
-EAGAIN and then rmnet_xmit() will return NETDEV_TX_BUSY to have the
TCP/IP stack reschedule the packet.  In the case where the high
watermark is hit, but not exceeded, then the function improperly returns
NETDEV_TX_BUSY resulting in the packet being sent a second time.  Since
the packets are deleted in the write-done notification, the second
transmit will result in either an access-after-delete or a double-delete
crash.

Remove check for high-water mark hit in the transmit function since it
is handled by a high-watermark event from SMUX already.

CRs-Fixed: 403691
Change-Id: Iadf3ac249d3fb8ccb378fc243415acbbfc3a1ff1
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>