nvmem: core: Fix msb clearing bits

When clearing the msb bits of the resultant buffer, it is
masked with the modulo of the number of bits needed with
respect to the BITS_PER_BYTE. To mask out the buffer,
it is passed though GENMASK of the remainder of the bits
starting from zeroth bit. This case is valid if nbits is not
a multiple of BITS_PER_BYTE and you are actually creating
a GENMASK. If the nbits coming is a multiple of BITS_PER_BYTE,
it would pass a negative value to the high bit number of
GENMASK with zero as the lower bit number.

As per the definition of the GENMASK, the higher bit number (h)
is right operand for bitwise right shift. If the value of the
right operand is negative or is greater or equal to the number
of bits in the promoted left operand, the behavior is undefined.
So passing a negative value to GENMASK could behave differently
across architecture, specifically between 64 and 32 bit.
Also, on passing the hard-coded negative value as GENMASK(-1, 0)
is giving compiler warning for shift-count-overflow.
Hence making a check for clearing the MSB if the nbits are not
a multiple of BITS_PER_BYTE.

Change-Id: Ic6edc4df87e5f52fdfe1ed8dad9029191b4739a5
Signed-off-by: Shadab Naseem <snaseem@codeaurora.org>