net/libertas: make SPI interface big endian aware

The comment (which I remove) says that the translation is done SPI routines.
IMHO this can't work because the SPI driver does not know whether the incomming
bytes are part of the registers/bytes which need to be flipped or part of
packet data which has to remain untouched.
While adding le helpers I also removed spu_write_u32() which has no users.

Tested-by: Andrey Yurovsky <andrey@cozybit.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>