lib: os: cast to the same size composite expression

In file crc16_sw.c essential type of LHS operand (16 bit) is wider than
essential type of composite expression in RHS operand (8 bit).
In crc32c_sw.c and crc32_sw.c Essential type of LHS operand (32 bit) is
wider than essential type of composite expression in RHS operand (8 bit)

Found as a coding guideline violation (MISRA R10.7) by static
coding scanning tool.

Signed-off-by: Maksim Masalski <maksim.masalski@intel.com>