atmel_serial: fix bad BUILD_BUG_ON() usage

is_power_of_2() appears not to be constant enough for BUILD_BUG_ON()
after the latest rework, so replace it with an open-coded test.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Jan Beulich <jbeulich@novell.com>
Cc: Haavard Skinnemoen <hskinnemoen@atmel.com>
Cc: Alan Cox <alan@linux.intel.com>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Claudio Scordino <claudio@evidence.eu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>