net: tsec: Fix mac addr setup portability, cleanup

Fix the 32-bit memory access that is not "endianess safe",
i.e. not giving the desired byte layout for LE cpus:
tempval = *((uint *) (tmpbuf + 4)), where 'char tmpbuf[]'.

Free the stack from rendundant local vars:
tmpbuf[] and i.

Use a portable type (u32) for the 32bit tsec register value
holder: tempval.

Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>