net: ipa: request GSI IRQ later

Introduce gsi_irq_init() and gsi_irq_exit(), to encapsulate looking
up the GSI IRQ and registering its handler.  Call gsi_irq_init() a
little later in gsi_init(), and initialize the completion earlier.
The IRQ handler accesses both the GSI virtual memory pointer and the
completion, and this way these things will have been initialized
before the gsi_irq() can ever be called.

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