atmel_lcdfb: fix pixclock divider calculation

Fix divider calculation and allow CLKVAL = 0 (divisor 2)

It was not possible to get the clock value 0 (divisor 2) because
the test "<=0" force the BYPASS bit to be activated instead.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Per Hedblom <per.hedblom@abem.se>
Cc: Roel Kluin <12o3l@tiscali.nl>
Cc: Jan Weber <jw022609@uni-greifswald.de>
Cc: Andrew Victor <linux@maxim.org.za>
Cc: Haavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>