ata: hpt366: fix incorrect mask when checking at cmd_high_time

According to the HPT366 data sheet, PCI config space dword 0x40-0x43
bits 11:8 specify the primary drive cmd_high_time, however,
currently just 3 bits of the 4 are being used because the mask
is 0x700 and not 0x0f00.  Fix the mask, allowing for the 40MHz clock
to be detected.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Tejun Heo <tj@kernel.org>