i2cset: printing help is not an error

The check distinguishing the 'h' and '?' parameters is dead code. Found
by cppcheck:

i2cset.c:177:4: warning: Statements following noreturn function 'help()' will never be executed. [unreachableCode]

Add a parameter to help saying how it should exit. Remove a forward
declaration while here.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>