Fix LinuxKPI regression after r321920. The mda_unit and si_drv0 fields are not
wide enough to hold the full 64-bit dev_t. Instead use the "dev" field in
the "linux_cdev" structure to store and lookup this value.

While at it remove superfluous use of parenthesis inside the
MAJOR(), MINOR() and MKDEV() macros in the LinuxKPI.

MFC after: 1 week
Sponsored by: Mellanox Technologies