pinctrl: aspeed: g6: Remove const specifier from aspeed_g6_sig_expr_set's ctx parameter

clang errors:

drivers/pinctrl/aspeed/pinctrl-aspeed-g6.c:2325:9: error: incompatible
pointer types initializing 'int (*)(struct aspeed_pinmux_data *, const
struct aspeed_sig_expr *, bool)' with an expression of type 'int (const
struct aspeed_pinmux_data *, const struct aspeed_sig_expr *, bool)'
[-Werror,-Wincompatible-pointer-types]
        .set = aspeed_g6_sig_expr_set,
               ^~~~~~~~~~~~~~~~~~~~~~
1 error generated.

Commit 674fa8da ("pinctrl: aspeed-g5: Delay acquisition of regmaps")
changed the set function pointer declaration and the g6 one wasn't
updated (I assume because it wasn't merged yet).

Fixes: 2eda1cde

 ("pinctrl: aspeed: Add AST2600 pinmux support")
Link: https://github.com/ClangBuiltLinux/linux/issues/632
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Link: https://lore.kernel.org/r/20190807003037.48457-1-natechancellor@gmail.com
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>