drivers: sensor: bmi160: Fix bug where undersampling is not set

Fix #32774

Bit position BIT(7) is 1000 0000 in binary which is actually the
bit positon 128. BIT(7) can be used as a mask, but we need to define
the position specifically as the integer 7.

Signed-off-by: Guðni Már Gilbert <gudni.m.g@gmail.com>