wl12xx: Fix validation of pm_runtime_get_sync return value

wl1271_sdio_power_on checks if the return value of pm_runtime_get_sync
is non-zero, and if so bails out.
However, pm_runtime_get_sync can return a positive number which does not
suggest an error has occurred. This is problematic for two reasons:

1. The function will needlessly bail out without decrementing back the
   runtime PM reference counter.
2. wl1271_power_on only checks if wl1271_power_on return value is
   negative. This means that wl1271_power_on will continue even if
   wl1271_sdio_power_on bailed out. As a result, sdio transactions will
   be initiated without properly enabling the sdio function and claiming
   the host. This could even lead to a kernel panic.

Fix this by only checking that the return value of pm_runtime_get_sync
is non-negative.

Signed-off-by: Ido Yariv <ido@wizery.com>
Acked-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>