x86/entry/32: Convert do_fast_syscall_32() to bool return type

Doesn't have to be 'long' - this simplifies the code a bit.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20230721161018.50214-5-brgerst@gmail.com