b43: Move code from nphy.* to phy_n.*

Sourcecode files for PHY code are named by phy_XXX.{c,h}
where XXX is the PHY type.
Move the N-PHY code to match the other files.

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