staging: et131x: Fix ET_INTR_TXDMA_ISR register name typo

We actually mean to clear the ET_INTR_TXDMA_ISR reg after handling
a completed transfer, not the ET_INTR_TXDMA_ERR reg, which should
get handled immediately after.

Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>