x86/asm/32: Fix regs_get_register() on segment registers

The segment register high words on x86_32 may contain garbage.
Teach regs_get_register() to read them as u16 instead of unsigned
long.

Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andy Lutomirski <luto@kernel.org>