staging: bcm2835-audio: Fix unused enable_headphones module parameter

Since commit "staging: bcm2835-audio: Add disable-headphones flag" the
enabling/disabling of the headphones output is solely determined by the
presence of the DT property 'brcm,disable-headphones' and the
enable_headphones module parameter is unused. Fix that by making it a
global switch.

Fixes: ee90e47d ("staging: bcm2835-audio: Add disable-headphones flag")
Signed-off-by: Juerg Haefliger <juergh@proton.me>