mwifiex: failure case handling for PCIe events

Event buffers for PCIe interface are allocated during driver
initialisation, and respective physical addresses are sent to FW
in *_PCIE_DESC_DETAILS command so that FW can do DMA. These buffers
will be freed while unloading the driver. Therefore we should not
free them in event handling error path. Also we should skip next
pending events in failure case.

Also fixed 'returning -1 instead of -ENOMEM is sloppy' warnings.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>