ssb: Make ssb_wait_bit multi-bit safe

ssb_wait_bit was designed for only one-bit bitmasks.
People start using it for multi-bit bitmasks. Make the "set" case
is safe for this. The "unset" case is already safe.

This does not change behavior of the current code.

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