b43: Fix MAC control and microcode init

This zeros out all microcode related memory before loading
the microcode.

This also fixes initialization of the MAC control register.
The _only_ place where we overwrite the contents of the MAC control
register is at the beginning of b43_chip_init().
All other places must do read() -> mask/set -> write() to not
overwrite existing bits.

This also adds a longer delay for waiting for the microcode
to initialize itself. It seems that the current timeout is sufficient
on all available devices, but there's no real reason why we shouldn't
wait for up to one second. Slow embedded devices might exist.
Better safe than sorry.

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