driver/Makefile: Initialize "mtd" and "spi" before "net"

On TI's da850/omap-l138 EVM, MAC address is stored in SPI flash.

This patch changes the initialization sequence of the drivers
by moving mtd and spi ahead of net in drivers/Makefile thereby
enabling da850/omap-l138 ethernet driver to read the MAC address
while booting.

Signed-off-by: Sudhakar Rajashekhara <sudhakar.raj@ti.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>