enc28j60: Fix reading of transmit status vector

This error was reported by cppcheck:
drivers/net/enc28j60.c:815: error: Using sizeof for array given as function argument returns the size of pointer.

The original code reads 4 or 8 bytes instead of TSV_SIZE (= 100) bytes.
I just fixed the code, but did not run any tests.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: David S. Miller <davem@davemloft.net>