spmi: pmic-arb: Remove checking opc value not less than 0

The opc parameter in pmic_arb_write_cmd() function is defined with type
u8 and it's always greater than or equal to 0. Checking that it's not
less than 0 is redundant and it can cause a forbidden warning during
compilation. Remove the check.

Change-Id: I34a271447b8458107239fb85cbf5cc669b51ff6e
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>