ipmi_mc: Fix the IPM_DEV_FWREV1_MAJOR_MASK

The IPM_DEV_FWREV1_MAJOR_MASK was incorrectly defined as 0x3F.
The spec indicates that the major firmware revision has 0~6 valid bits,
so the mask shall be 0x7F.

Tested: Verify a BMC that has a major version that is large than 0x3F,
        and ipmitool shows the correct major version.

Signed-off-by: Lei YU <yulei.sh@bytedance.com>