mfd: ab8500-debugfs: Use common error handling code in ab8500_print_modem_registers()

Add jump targets so that two error messages are stored only once
at the end of this function implementation.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>