[ARM] pxa/csb726: adjust duplicate structure field initialization

Currently the irq_type field of the csb726_lan_config structure is
initialized twice.  The value in the first case,
SMSC911X_IRQ_POLARITY_ACTIVE_LOW, is normally stored in the irq_polarity
field, so I have renamed the field in the first initialization to that.

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>