common/cmd_i2c.c: Fix GCC 4.6 build warning

Fix:
cmd_i2c.c: In function 'do_i2c_add_bus':
cmd_i2c.c:1212:19: warning: variable 'dev' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Heiko Schocher <hs@denx.de>