drivers: i2c: stm32f4: Add check for BTF flag before reading N-2 byte

According to STM32F4 reference manual, software should wait for BTF=1
before reading N-2 data byte.

Reference:

"For N >2 -byte reception, from N-2 data reception" section, page
853 of STM32F4 Reference manual.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>