scripts/config: Fix wrong "shift" for --keep-case

Remove wrong "shift" for --keep-case. There is always "shift" at
beginning of while-loop. No need "shift" at --keep-case just before
"continue" to process next argument.

Now the following works as expected:

./scripts/config -e aAa -k -e bBb -e cCc && tail -3 .config
CONFIG_AAA=y
CONFIG_bBb=y
CONFIG_cCc=y

Signed-off-by: Hiroshi Doyu <hdoyu@nvidia.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>