b43: Always use block-I/O for the PIO data registers

On SDIO the PIO data register seems to be hardwired to LE. So
the MACCTL bit has no effect on the endianness.
So also use block-I/O for the last word of the packet. block-I/O is always LE.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Tested-by: Albert Herranz <albert_herranz@yahoo.es>
Signed-off-by: John W. Linville <linville@tuxdriver.com>