wl1271: Clean up firmware block allocation calculation

This patch simplifies the required block count calculation. Though it
introduces a division operator, it greatly simplifies the formula.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Teemu Paasikivi <ext-teemu.3.paasikivi@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>