bcma: fix incorrect update of BCMA_CORE_PCI_MDIO_DATA

An earlier commit re-worked the setting of the bitmask and is now
assigning v with some bit flags rather than bitwise or-ing them
into v, consequently the earlier bit-settings of v are being lost.
Fix this by replacing an assignment with the bitwise or instead.

Addresses-Coverity: ("Unused value")
Fixes: 2be25cac ("bcma: add constants for PCI and use them")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>