hw/net/eepro100: Fix endianness problem on big endian hosts

Since commit 1865e288 ("Fix eepro100 simple transmission
mode"), the test/pxe-test is broken for the eepro100 device on big
endian hosts. However, it seems like that commit did not introduce the
problem, but just uncovered it: The EEPRO100State->tx.tbd_array_addr and
EEPRO100State->tx.tcb_bytes fields are already in host byte order, since
they have already been byte-swapped in the read_cb() function.
Thus byte-swapping them in tx_command() again results in the wrong
endianness. Removing the byte-swapping here fixes the pxe-test.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>