scripts/gdb: clean up error handling in list helpers

An incorrect argument to list_for_each is an internal error in gdb
scripts so a TypeError should be raised.  The gdb.GdbError exception
type is intended for user errors such as incorrect invocation.

Drop the type assertion in list_for_each_entry because list_for_each
isn't going to suddenly yield something else.

Applies to both list and hlist

Link: http://lkml.kernel.org/r/c1d3fd4db13d999a3ba57f5bbc1924862d824f61.1556881728.git.leonard.crestez@nxp.com
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Jason Wessel <jason.wessel@windriver.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>