pinctrl: aspeed: fix regmap error handling

The newly added aspeed driver tries to check for a negative return
value from a pinctrl function, but stores the intermediate value in
a 'bool' variable, which cannot work:

drivers/pinctrl/aspeed/pinctrl-aspeed.c: In function 'aspeed_sig_expr_set':
drivers/pinctrl/aspeed/pinctrl-aspeed.c:192:11: error: comparison of constant '0' with boolean expression is always false [-Werror=bool-compare]

This slightly reworks the logic to use an explicit comparison with zero
before assigning to the temporary variable.

Reported-by: Colin King <colin.king@canonical.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>