ath6kl: Unify sg_sz and buf_sz in ath6kl_sdio_alloc_prep_scat_req()

sg_sz and buf_sz are initialized and used in a mutual exclusive way.
However, some versions of gcc are not smart enough to see this:

drivers/net/wireless/ath/ath6kl/sdio.c: In function ‘ath6kl_sdio_alloc_prep_scat_req’:
drivers/net/wireless/ath/ath6kl/sdio.c:338: warning: ‘sg_sz’ may be used uninitialized in this function

Unify the sg_sz and buf_sz variables into a single size variable to kill
the compiler warning.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>