wlcore: Fix regression in wlcore_set_partition()

The commit 3719c17e ("wlcore/wl18xx: fw logger over sdio") introduced a
regression causing the wlcore to time out and go into recovery. Reverting the
changes regarding write of the last partition size brings the module back to
it's functional state.

Fixes: 3719c17e

 ("wlcore/wl18xx: fw logger over sdio")
Reported-by: Ross Green <rgkernel@gmail.com>
Signed-off-by: Emil Goode <emil.fsw@goode.io>
[kvalo@codeaurora.org: improved commit log]
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>