i2cdump: don't use common_bufsiz1

i2cdump: don't use common_bufsiz1

Commit e6a2f4cc changed the way common_bufsiz1 works. Now it needs to
be initialized before using, but i2cdump wasn't updated by said patch.

Since the fact that we're using common_bufsiz1 here isn't obvious (no
G_INIT() macro, no other global variables), drop it and simply
allocate the integer array required for block reads on the stack.

Tested with i2c block read on a Lenovo Thinkpad laptop.

Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>