mtd: quiet sparse noise in cfi.h

In the inline function cfi_build_cmd_addr, the cast of cmd_ofs to an
uint8_t produces a sparse warning of the type:

warning: cast truncates bits from constant value (2aa becomes aa)

Quiet the warning by masking cmd_ofs with 0xff and remove the cast.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>