iio:pressure:bmp280: fix pressure calculation

According to the datasheet and as defined in struct bmp280_comp_press, dig_p1 is
of type u16, while dig_p2 to dig_p9 are of type s16. In
bmp280_read_compensation_press(), values read from the device were treated as
the wrong type.
In bmp280_read_press() the fractional part of the measured pressure is
calculated wrong. A better way is to use *val for the raw pressure and *val2 for
the quotient and let the core do the proper conversion using IIO_VAL_FRACTIONAL.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Acked-by: Vlad Dogaru <vlad.dogaru@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>