drivers: net: cpsw: Fix TX_IN_SEL offset

The TX_IN_SEL offset for the CPSW_PORT/TX_IN_CTL register was
incorrect. This caused the Dual MAC mode to never get set when
it should. It also caused possible unintentional setting of a
bit in the CPSW_PORT/TX_BLKS_REM register.

The purpose of setting the Dual MAC mode for this register is to:

    "... allow packets from both ethernet ports to be written into
     the FIFO without one port starving the other port."
					- AM335x ARM TRM

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>