b43: N-PHY: fix gain in b43_nphy_get_gain_ctl_workaround_ent()

There were no break statements in this switch statement so everything
used the default settings.  Per Walter Harms's suggestion, I've replaced
the switch statement and done a little cleanup.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>