usb: f_mass_storage: Fix set_bit and clear_bit usage

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

Looking at the provenance of the current u-boot code and the git change
history in the kernel, it looks like we have a local copy of set_bit and
clear_bit as a hold-over from porting the Linux driver into u-boot.

These days __set_bit and __clear_bit are optionally provided by an arch and
can be used as inputs to generic_bit_set and generic_bit_clear.

This patch switches over to generic_set_bit and generic_clear_bit to
accommodate.

Tested on i.MX WaRP7 and Intel Edison

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