ath6kl: use CONFIG_ATH6KL_ENABLE_COEXISTENCE

When CONFIG_ATH6KL_ENABLE_COEXISTENCE gets enabled we define
another flag called ENABLE_COEXISTENCE, just use
CONFIG_ATH6KL_ENABLE_COEXISTENCE since that is already defined
by the generated autoconf header file from the kernel.

Cc: Naveen Singh <nsingh@atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>