usb: composite convert __set_bit to generic_set_bit

Compiling the f_mass_storage driver for an x86 target results in a
compilation error as set_bit and clear_bit are provided by bitops.h

To address that situation we discussed on the list moving to
genetic_set_bit() instead.

Doing a quick grep for similar situations in drivers/usb shows that the
composite device is using __set_bit().

This patch switches over to generic_set_bit to maintain consistency between
the two gadget drivers.

Signed-off-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Cc: Lukasz Majewski <lukma@denx.de>
Cc: Marek Vasut <marex@denx.de>