tests/tinycrypt: Fix wrong sizeof argument in test_ccm_mode (2nd)

This commit fixes an issue in the test_ccm_mode.c file:

sizeof(data) is used to compute the length of the array pointed to
by the 'uint8_t *data' pointer.

At the same function scope, there is a variable (dlen) that already
specifies the required length, so we use that variable instead of
the 'sizeof' function call.

This issue was not reported by Coverity, although is worth to fix it.

Change-Id: I27cbf8c7000a4189a42d193f6445996d4b852aa6
Signed-off-by: Flavio Santes <flavio.santes@intel.com>