spmi: pmic_arb: Fix opcode calculate error in pmic_arb_cmd

The original calculation of spmi command in pmic_arb_cmd()function
has wrong opcode bits value.

Change-Id: Ifdddaac4d4a58b71dca8307af002435333af0910
Signed-off-by: Wu Fenglin <fenglinw@codeaurora.org>