ath9k: Fix TX99 config option usage

ath9k: Fix TX99 config option usage

Use CONFIG_ATH9K_TX99 to properly enclose the tx99 code
and make sure that it is not compiled as part of the driver
when it is not selected. Move the tx99 code to a new file tx99.c
and also add ATH9K_DEBUGFS as a dependency in Kconfig.

This reduces the module size on platforms like OpenWrt where
ATH9K_DEBUGFS is selected, but TX99 might be disabled.

Cc: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>