Bluetooth: Kconfig: Increase default RX stack size for LE Audio

LE Audio builds on top of the BT Host stack, and will
thus require a higher amount of stack size. Even simple
applications using BAP will likely reach the 1024 default
size with the default BAP configurations, and when
we start adding CAP and even TMAP/HAP on top of it, it
will likely increase even further.

The default value of 2048 is unlikely to be reached,
and applications that want to optimize can likely
reduce it, depending on the configuration.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>