rtlbtfw: handle partial reads when loading firmware

[chris.longros_gmail.com]

Summary

Loop until all bytes are read or a real error occurs, instead of
assuming a single read() returns the entire file. Retry on EINTR
and break on EOF. Use intmax_t casts in the error message to
match off_t portably.

Signed-off-by: Christos Longros

[wulf]

Although it looks good, replacing read() with mmap() -> memcpy() -> munmap() sequence would make code simpler and cleaner

[wulf]

I committed mmap()-based changes. See: https://cgit.freebsd.org/src/commit/?id=87ed6840a0a6320295f6abb43a987d20ae126cf7

[chris.longros_gmail.com]

In D56500#1296504, @wulf wrote:

I committed mmap()-based changes. See: https://cgit.freebsd.org/src/commit/?id=87ed6840a0a6320295f6abb43a987d20ae126cf7

I was working on the changes, but thanks for implementing them :)