drivers: net: cpsw: fix disabling of tx interrupt in rx isr

In commit 'c03abd84 ("net: ethernet: cpsw: don't requests
IRQs we don't use")', common isr is split into tx and rx, but
in rx isr tx interrupt is also disabledi in cpsw_disable_irq().
So tx interrupts are not handled during rx interrupts and rx
napi completion and results in poor tx performance by 40Mbps.
Fixing by disabling only rx interrupt in rx isr.

Cc: Felipe Balbi <balbi@ti.com>
Cc: <stable@vger.kernel.org> # v4.0+
Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>