post/drivers/i2c.c: fix compile error

Commit 7e263cea "post/i2c: Clean up detection logic" added a "const"
qualifier to the declaration of i2c_addr_list[], missing the fact that
the list gets modified later in the code, which results in build
errors like these:

i2c.c: In function 'i2c_post_test':
i2c.c:88: error: assignment of read-only location

Remove the incorrect "const".

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Peter Tyser <ptyser@xes-inc.com>
Cc: Heiko Schocher <hs@denx.de>
Acked-by: Heiko Schocher <hs@denx.de>