wl1271: Implement looped IRQ handling

This patch implements looped IRQ handling. In essence, if a new interrupt is
asserted by the FW while the host is processing the previous one, the host
will directly proceed processing the new IRQ without leaving the handling
function.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Teemu Paasikivi <ext-teemu.3.paasikivi@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>