bcma: Move initialization of SPROM to prevent overwrite

The first thing bcm47xx_fill_sprom does is initialize (zero fill) the SPROM.  For
BCMA SOC, this wipes out any values previously read by bcm47xx_fill_sprom_ethernet
(see arch/mips/bcm47xx/setup.c - bcm47xx_get_sprom_bcma).  Move the initialization
of SPROM so it is called prior to filling in any values.

Signed-off-by: Nathan Hintz <nlhintz@hotmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>