atmel_serial: filter out FP during baud rate detection

I made a change to u-boot that used the FP (Fractional Part) field of BRGR
to achieve more accurate baud rate generation.  Unfortunately, the
atmel_serial driver looks at the whole BRGR register when trying to detect
the baud rate that the port is currently running at, so setting FP to a
nonzero value breaks the baud rate detection.

I'll sit on the u-boot patch for a while longer, but this is clearly a
bug in the atmel_serial driver which should be fixed.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Acked-by: Andrew Victor <linux@maxim.org.za>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>