wlcore: fix error handling in wlcore_event_fw_logger

wlcore_read/wlcore_write can return negative values so it should
be assigned to signed variable.

The problem has been detected using proposed semantic patch
scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci [1].

[1]: http://permalink.gmane.org/gmane.linux.kernel/2120705



Fixes: 3719c17e ("wlcore/wl18xx: fw logger over sdio")
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>