stm32mp1: bsec: Fix warnings when compiling with W=1

stm32mp1: bsec: Fix warnings when compiling with W=1

This patch solves the following warnings:

arch/arm/mach-stm32mp/bsec.c: In function 'stm32mp_bsec_read':
warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  if (offset >= STM32_BSEC_OTP_OFFSET) {
             ^~
arch/arm/mach-stm32mp/bsec.c: In function 'stm32mp_bsec_write':
warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  if (offset >= STM32_BSEC_OTP_OFFSET) {
             ^~
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>