b43: Use SSB block-I/O to do PIO

This changes the b43-PIO code to use the new SSB block-I/O.
This reduces the overhead by removing lots of function calls, pointer
dereferencing, if-conditionals any byteswapping for each packet data word.

This also fixes a harmless sparse endianness warning.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>