net: ipa: move GSI command opcode values into "gsi_reg.h"

The gsi_ch_cmd_opcode, gsi_evt_cmd_opcode, and gsi_generic_cmd_opcode
enumerated types are values that fields in the GSI command registers
can take on.  Move their definitions out of "gsi.c" and into "gsi_reg.h",
alongside the definition of registers they are associated with.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>