PCI: fix return value from pcix_get_max_mmrbc()

For the PCI_X_STATUS register, pcix_get_max_mmrbc() is returning an incorrect
value, which is based on:

	(stat & PCI_X_STATUS_MAX_READ) >> 12

Valid return values are 512, 1024, 2048, 4096, which correspond to a 'stat'
(masked and right shifted by 21) of 0, 1, 2, 3, respectively.

A right shift by 11 would generate the correct return value when 'stat' (masked
and right shifted by 21) has a value of 1 or 2. But for a value of 0 or 3 it's
not possible to generate the correct return value by only right shifting.

Fix is based on pcix_get_mmrbc()'s similar dealings with the PCI_X_CMD register.

Cc: stable@kernel.org
Signed-off-by: Dean Nelson <dnelson@redhat.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>