bswap: Modify prototype of stb_p (avoid type conversions)

bswap: Modify prototype of stb_p (avoid type conversions)

The function uses an uint8_t value, so show this in the function
prototype. Non-optimizing compilers will avoid unnecessary type
conversions from (u)int8_t to int and back to uint8_t when generating
calls of this inline function.

stw_p, stl_p and stq_p already use similar prototypes.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>