KVM: PPC: Book3S HV: Fix mask size for emulated msgsndp

According to ISAv3.1 and ISAv3.0b, the msgsndp is described to split
RB in:
  msgtype <- (RB) 32:36
  payload <- (RB) 37:63
  t       <- (RB) 57:63

The current way of getting 'msgtype', and 't' is missing their MSB:
  msgtype: ((arg >> 27) & 0xf) : Gets (RB) 33:36, missing bit 32
  t:       (arg &= 0x3f)       : Gets (RB) 58:63, missing bit 57

Fixes this by applying the correct mask.

Signed-off-by: Leonardo Bras <leobras.c@gmail.com>
Acked-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201208215707.31149-1-leobras.c@gmail.com