gpu: nvgpu: MISRA 10.3-Conversions to/from an enum

Fix violations where the conversion is from a non-enum type to enum
type or vice-versa.

JIRA NVGPU-659

Change-Id: I45f43c907b810cc86b2a4480809d0c6757ed3486
Signed-off-by: Amulya <Amurthyreddy@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/1802322
GVS: Gerrit_Virtual_Submit
Tested-by: Amulya Murthyreddy <amurthyreddy@nvidia.com>
Reviewed-by: Alex Waterman <alexw@nvidia.com>
Reviewed-by: Adeel Raza <araza@nvidia.com>
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

drivers/gpu/nvgpu/gk20a/fifo_gk20a.c

@@ -2720,7 +2720,7 @@ void gk20a_fifo_isr(struct gk20a *g)
 	return;
 }
 
-u32 gk20a_fifo_nonstall_isr(struct gk20a *g)
+enum gk20a_nonstall_ops gk20a_fifo_nonstall_isr(struct gk20a *g)
 {
 	u32 fifo_intr = gk20a_readl(g, fifo_intr_0_r());
 	u32 clear_intr = 0;