ntb: ntb_hw_intel: Skylake doorbells should be 32bits, not 64bits

Fixing doorbell register length to 32bits per spec. On Skylake NTB, the
doorbell registers are 32bit write only registers. The source for the
doorbell is a 64bit register that shows the interrupt bits.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Fixes: 783dfa6c ("ntb: Adding Skylake Xeon NTB support")
Acked-by: Allen Hubbe <Allen.Hubbe@dell.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>