b43: LP-PHY: Refactor TX gain table I/O

Make it possible to write individual gain table entries.
Allow gain table entries to be written outside gain table init.
Add version-agnostic helpers for writing gain tables.
Use the new TX gain table helpers during table init.

Signed-off-by: Gábor Stefanik <netrolller.3d@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>